Merge pull request #26016 from nazem0/dev

Add IQueryable projection support to read-only application services

docs/en/framework/architecture/domain-driven-design/application-services.md

@@ -444,6 +444,7 @@ These methods are low level methods that can control how to query entities from
 * `ApplyPaging` is used to make paging on the query. If your `TGetListInput` already implements `IPagedResultRequest`, you don't need to override this since the ABP automatically understands it and performs the paging.
 * `ApplySorting` is used to sort (order by...) the query. If your `TGetListInput` already implements the `ISortedResultRequest`, ABP automatically sorts the query. If not, it fallbacks to the `ApplyDefaultSorting` which tries to sort by creation time, if your entity implements the standard `IHasCreationTime` interface.
 * `GetEntityByIdAsync` is used to get an entity by id, which calls `Repository.GetAsync(id)` by default.
+* `CreateEntityQueryOrNullAsync` is used to create a query for a single entity by id, which is only needed for the *Query Projection* explained below. It returns `null` if the application service can not create such a query, then `GetEntityByIdAsync` is used.
 * `DeleteByIdAsync` is used to delete an entity by id, which calls `Repository.DeleteAsync(id)` by default.
 
 #### Object to Object Mapping
@@ -456,6 +457,103 @@ These methods are used to convert Entities to DTOs and vice verse. They use the
   * `MapToEntityAsync(TCreateInput)` is used to create an entity from `TCreateInput`.
   * `MapToEntityAsync(TUpdateInput, TEntity)` is used to update an existing entity from `TUpdateInput`.
 
+#### Query Projection
+
+`GetAsync` and `GetListAsync` get the entities from the database, then map them to DTOs in the memory. If your DTO uses only a few properties of a large entity, you can project the query to the DTO instead, so the database returns only the columns you need.
+
+Implement the `IQueryProjector<TEntity, TDto>` interface to define a projection:
+
+````csharp
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+
+namespace MyProject.Books;
+
+public class BookProjector : IQueryProjector<Book, BookDto>
+{
+    public IQueryable<BookDto> ProjectTo(IQueryable<Book> source)
+    {
+        return source.Select(book => new BookDto
+        {
+            Id = book.Id,
+            Name = book.Name
+        });
+    }
+}
+````
+
+You don't have to write the `Select` by hand. Both [Mapperly](https://mapperly.riok.app/) and [AutoMapper](https://docs.automapper.org) can project an `IQueryable`, refer to their own documentation for it and to the [object to object mapping document](../../infrastructure/object-to-object-mapping.md) for their ABP integrations. Your existing maps are not used for the projection, a projector is always a class implementing `IQueryProjector<TSource, TDestination>`.
+
+ABP registers the projectors by convention, you don't need to configure anything else. Implement a projector once for an entity and DTO pair, and use the `ReplaceServices` option of the `DependencyAttribute` to replace an existing one. Filters (like soft delete and multi-tenancy), sorting and paging are still applied to the query before the projection.
+
+> A projection must return one row per entity. The total count and the paging are calculated on the entity query before the projection runs, so a projection that filters out rows (an inner join to an optional relation) or multiplies them (a join to a collection) returns a page that doesn't match the reported total count. Use a left join for optional relations.
+
+The projector is synchronous, so it can not obtain the query of another aggregate root, which is only
+available through the asynchronous `GetQueryableAsync`. Override `CreateGetOutputDtoQueryOrNullAsync` or
+`CreateGetListOutputDtoQueryOrNullAsync` for that. They replace the projector for that application service:
+
+````csharp
+public class BookAppService : ReadOnlyAppService<Book, BookDto, Guid>
+{
+    private readonly IBookDtoQuery _bookDtoQuery;
+
+    //...
+
+    protected override async Task<IQueryable<BookDto>?> CreateGetListOutputDtoQueryOrNullAsync(IQueryable<Book> query)
+    {
+        return await _bookDtoQuery.ProjectAsync(query);
+    }
+}
+
+//The projection is a class of its own, so the other application services returning a BookDto reuse it
+public class BookDtoQuery : IBookDtoQuery, ITransientDependency
+{
+    private readonly IReadOnlyRepository<Author, Guid> _authorRepository;
+
+    //...
+
+    public async Task<IQueryable<BookDto>> ProjectAsync(IQueryable<Book> books)
+    {
+        var authors = await _authorRepository.GetQueryableAsync();
+
+        return from book in books
+               join author in authors on book.AuthorId equals author.Id into bookAuthors
+               from bookAuthor in bookAuthors.DefaultIfEmpty()
+               select new BookDto
+               {
+                   Id = book.Id,
+                   Name = book.Name,
+                   AuthorName = bookAuthor != null ? bookAuthor.Name : null
+               };
+    }
+}
+````
+
+Both queries must come from the same database context, otherwise they can not be executed as a single query,
+and the provider has to be able to translate the join. The one row per entity rule above applies here too,
+that's why the example uses a left join. A joined column can not be used for the sorting, and the paging is
+based on the entity query, since both are applied before this method is called.
+
+A projector is resolved by the `(entity, DTO)` type pair, just like an `IObjectMapper<TSource, TDestination>`, so registering one enables the projection for every application service using that pair. It replaces the way the DTOs are read:
+
+* `GetListAsync` doesn't use `MapToGetListOutputDtosAsync` anymore.
+* `GetAsync` doesn't use `GetEntityByIdAsync` and `MapToGetOutputDtoAsync` anymore, as long as the application service can create a query for a single entity. `ReadOnlyAppService` and `CrudAppService` already do that. A class deriving from `AbstractKeyReadOnlyAppService` has to override `CreateEntityQueryOrNullAsync`, otherwise `GetAsync` keeps loading the entity and mapping it.
+
+The rest of the pipeline is untouched. The authorization policies are still checked, `CreateFilteredQueryAsync`, `ApplySorting` and `ApplyPaging` are still used, the data filters (like soft delete and multi-tenancy) are still applied, and the create, update and delete methods still use the [IObjectMapper](../../infrastructure/object-to-object-mapping.md).
+
+> If an application service needs to keep using the entity based extension points, override the `GetOutputDtoQueryProjector` or `GetListOutputDtoQueryProjector` property and return `null`:
+
+````csharp
+public class BookAppService : CrudAppService<Book, BookDto, Guid>
+{
+    protected override IQueryProjector<Book, BookDto>? GetOutputDtoQueryProjector => null;
+
+    protected override IQueryProjector<Book, BookDto>? GetListOutputDtoQueryProjector => null;
+
+    //...
+}
+````
+
 ## Miscellaneous
 
 ### Working with Streams

framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyReadOnlyAppService.cs

@@ -2,12 +2,14 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Linq.Dynamic.Core;
+using System.Threading;
 using System.Threading.Tasks;
 using Volo.Abp.Application.Dtos;
 using Volo.Abp.Auditing;
 using Volo.Abp.Domain.Entities;
 using Volo.Abp.Domain.Repositories;
 using Volo.Abp.ObjectMapping;
+using Volo.Abp.Threading;
 
 namespace Volo.Abp.Application.Services;
 
@@ -44,6 +46,20 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
 
     protected virtual string? GetListPolicyName { get; set; }
 
+    /// <summary>
+    /// Used by the <see cref="CreateGetOutputDtoQueryOrNullAsync"/> to project the query to the <typeparamref name="TGetOutputDto"/>.
+    /// The <see cref="GetEntityByIdAsync"/> and the <see cref="MapToGetOutputDtoAsync"/> are not used while the query is projected.
+    /// </summary>
+    protected virtual IQueryProjector<TEntity, TGetOutputDto>? GetOutputDtoQueryProjector
+        => LazyServiceProvider.LazyGetService<IQueryProjector<TEntity, TGetOutputDto>>();
+
+    /// <summary>
+    /// Used by the <see cref="CreateGetListOutputDtoQueryOrNullAsync"/> to project the query to the <typeparamref name="TGetListOutputDto"/>.
+    /// The <see cref="MapToGetListOutputDtosAsync"/> is not used while the query is projected.
+    /// </summary>
+    protected virtual IQueryProjector<TEntity, TGetListOutputDto>? GetListOutputDtoQueryProjector
+        => LazyServiceProvider.LazyGetService<IQueryProjector<TEntity, TGetListOutputDto>>();
+
     protected AbstractKeyReadOnlyAppService(IReadOnlyRepository<TEntity> repository)
     {
         ReadOnlyRepository = repository;
@@ -53,6 +69,19 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
     {
         await CheckGetPolicyAsync();
 
+        var dtoQuery = await CreateGetOutputDtoQueryOrNullAsync(id);
+        if (dtoQuery != null)
+        {
+            //TGetOutputDto has no class constraint, so a default value can not be used to detect the missing entity
+            var dtos = await AsyncExecuter.ToListAsync(dtoQuery.Take(1), GetCancellationToken());
+            if (dtos.Count == 0)
+            {
+                throw new EntityNotFoundException<TEntity>(id);
+            }
+
+            return dtos[0];
+        }
+
         var entity = await GetEntityByIdAsync(id);
 
         return await MapToGetOutputDtoAsync(entity);
@@ -65,7 +94,6 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
         var query = await CreateFilteredQueryAsync(input);
         var totalCount = await AsyncExecuter.CountAsync(query);
 
-        var entities = new List<TEntity>();
         var entityDtos = new List<TGetListOutputDto>();
 
         if (totalCount > 0)
@@ -73,8 +101,16 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
             query = ApplySorting(query, input);
             query = ApplyPaging(query, input);
 
-            entities = await AsyncExecuter.ToListAsync(query);
-            entityDtos = await MapToGetListOutputDtosAsync(entities);
+            var dtoQuery = await CreateGetListOutputDtoQueryOrNullAsync(query);
+            if (dtoQuery != null)
+            {
+                entityDtos = await AsyncExecuter.ToListAsync(dtoQuery);
+            }
+            else
+            {
+                var entities = await AsyncExecuter.ToListAsync(query);
+                entityDtos = await MapToGetListOutputDtosAsync(entities);
+            }
         }
 
         return new PagedResultDto<TGetListOutputDto>(
@@ -85,6 +121,57 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
 
     protected abstract Task<TEntity> GetEntityByIdAsync(TKey id);
 
+    private CancellationToken GetCancellationToken()
+    {
+        return LazyServiceProvider
+            .LazyGetService<ICancellationTokenProvider>(NullCancellationTokenProvider.Instance)
+            .FallbackToProvider();
+    }
+
+    /// <summary>
+    /// Should create a query that selects the entity with the given <paramref name="id"/>.
+    /// It returns null by default, then the entity is not projected.
+    /// </summary>
+    /// <param name="id">The id of the entity.</param>
+    protected virtual Task<IQueryable<TEntity>?> CreateEntityQueryOrNullAsync(TKey id)
+    {
+        return Task.FromResult<IQueryable<TEntity>?>(null);
+    }
+
+    /// <summary>
+    /// Projects the query of the entity with the given <paramref name="id"/> to the <typeparamref name="TGetOutputDto"/>.
+    /// It uses the <see cref="GetOutputDtoQueryProjector"/> and the <see cref="CreateEntityQueryOrNullAsync"/> by default,
+    /// and the <see cref="GetEntityByIdAsync"/> is used when it returns null.
+    /// Override it to await other queries, like the query of another aggregate root to join.
+    /// </summary>
+    /// <param name="id">The id of the entity.</param>
+    protected virtual async Task<IQueryable<TGetOutputDto>?> CreateGetOutputDtoQueryOrNullAsync(TKey id)
+    {
+        var queryProjector = GetOutputDtoQueryProjector;
+        if (queryProjector == null)
+        {
+            return null;
+        }
+
+        var query = await CreateEntityQueryOrNullAsync(id);
+
+        return query == null ? null : queryProjector.ProjectTo(query);
+    }
+
+    /// <summary>
+    /// Projects the given entity query to the <typeparamref name="TGetListOutputDto"/>.
+    /// It uses the <see cref="GetListOutputDtoQueryProjector"/> by default,
+    /// and the <see cref="MapToGetListOutputDtosAsync"/> is used when it returns null.
+    /// Override it to await other queries, like the query of another aggregate root to join.
+    /// The projection must return one row per entity: the total count is already calculated and the paging is
+    /// already applied, so adding or removing rows makes the page inconsistent with the total count.
+    /// </summary>
+    /// <param name="query">The sorted and paged entity query.</param>
+    protected virtual Task<IQueryable<TGetListOutputDto>?> CreateGetListOutputDtoQueryOrNullAsync(IQueryable<TEntity> query)
+    {
+        return Task.FromResult(GetListOutputDtoQueryProjector?.ProjectTo(query));
+    }
+
     protected virtual async Task CheckGetPolicyAsync()
     {
         await CheckPolicyAsync(GetPolicyName);

framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/CrudAppService.cs

@@ -84,6 +84,13 @@ public abstract class CrudAppService<TEntity, TGetOutputDto, TGetListOutputDto,
         return await Repository.GetAsync(id);
     }
 
+    protected override async Task<IQueryable<TEntity>?> CreateEntityQueryOrNullAsync(TKey id)
+    {
+        var query = await Repository.GetQueryableAsync();
+
+        return query.Where(e => e.Id!.Equals(id));
+    }
+
     protected override void MapToEntity(TUpdateInput updateInput, TEntity entity)
     {
         if (updateInput is IEntityDto<TKey> entityDto)

framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/ReadOnlyAppService.cs

@@ -47,6 +47,13 @@ public abstract class ReadOnlyAppService<TEntity, TGetOutputDto, TGetListOutputD
         return await Repository.GetAsync(id);
     }
 
+    protected override async Task<IQueryable<TEntity>?> CreateEntityQueryOrNullAsync(TKey id)
+    {
+        var query = await Repository.GetQueryableAsync();
+
+        return query.Where(e => e.Id!.Equals(id));
+    }
+
     protected override IQueryable<TEntity> ApplyDefaultSorting(IQueryable<TEntity> query)
     {
         if (typeof(TEntity).IsAssignableTo<ICreationAuditedObject>())

framework/src/Volo.Abp.ObjectMapping/Volo/Abp/ObjectMapping/AbpObjectMappingModule.cs

@@ -1,4 +1,5 @@
-using Microsoft.Extensions.DependencyInjection;
+using System.Collections.Generic;
+using Microsoft.Extensions.DependencyInjection;
 using Volo.Abp.DependencyInjection;
 using Volo.Abp.Modularity;
 using Volo.Abp.Reflection;
@@ -18,6 +19,14 @@ public class AbpObjectMappingModule : AbpModule
                     typeof(IObjectMapper<,>)
                 ).ConvertAll(t => new ServiceIdentifier(t))
             );
+
+            //Register types for IQueryProjector<TSource, TDestination> if implements
+            foreach (var serviceType in ReflectionHelper.GetImplementedGenericTypes(
+                         onServiceExposingContext.ImplementationType,
+                         typeof(IQueryProjector<,>)))
+            {
+                onServiceExposingContext.ExposedTypes.AddIfNotContains(new ServiceIdentifier(serviceType));
+            }
         });
     }
 

framework/src/Volo.Abp.ObjectMapping/Volo/Abp/ObjectMapping/IQueryProjector.cs

@@ -0,0 +1,24 @@
+using System.Linq;
+using Volo.Abp.DependencyInjection;
+
+namespace Volo.Abp.ObjectMapping;
+
+/// <summary>
+/// Maps a query to another.
+/// Implement this interface to project a query on the data store side, instead of loading the
+/// source objects into the memory and mapping them one by one.
+/// Implement it once for a source and destination pair. Use the ReplaceServices option of the
+/// DependencyAttribute to replace an existing implementation.
+/// </summary>
+/// <typeparam name="TSource">Type of the source objects</typeparam>
+/// <typeparam name="TDestination">Type of the destination objects</typeparam>
+public interface IQueryProjector<TSource, TDestination> : ITransientDependency
+{
+    /// <summary>
+    /// Projects the given query. The returned query must be built on top of it and must keep its order,
+    /// with a single destination object for each source object, using expressions the query provider can
+    /// translate. The caller may have already sorted, paged or counted the source query.
+    /// </summary>
+    /// <param name="source">The query to project</param>
+    IQueryable<TDestination> ProjectTo(IQueryable<TSource> source);
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/Book.cs

@@ -0,0 +1,23 @@
+using System;
+using Volo.Abp.Domain.Entities;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class Book : Entity<Guid>
+{
+    public string Name { get; set; } = default!;
+
+    public int Price { get; set; }
+
+    public Book()
+    {
+
+    }
+
+    public Book(Guid id, string name, int price)
+        : base(id)
+    {
+        Name = name;
+        Price = price;
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookAbstractKeyAppService.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookAbstractKeyAppService : AbstractKeyReadOnlyAppService<Book, BookDto, Guid>
+{
+    public BookAbstractKeyAppService(IReadOnlyRepository<Book> repository)
+        : base(repository)
+    {
+
+    }
+
+    protected override async Task<Book> GetEntityByIdAsync(Guid id)
+    {
+        var query = await ReadOnlyRepository.GetQueryableAsync();
+
+        return await AsyncExecuter.FirstAsync(query, book => book.Id == id);
+    }
+
+    protected override IQueryable<Book> ApplyDefaultSorting(IQueryable<Book> query)
+    {
+        return query.OrderBy(book => book.Id);
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookAbstractKeyProjectingAppService.cs

@@ -0,0 +1,35 @@
+#nullable enable
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookAbstractKeyProjectingAppService : AbstractKeyReadOnlyAppService<Book, BookDto, Guid>
+{
+    public BookAbstractKeyProjectingAppService(IReadOnlyRepository<Book> repository)
+        : base(repository)
+    {
+
+    }
+
+    protected override async Task<Book> GetEntityByIdAsync(Guid id)
+    {
+        var query = await ReadOnlyRepository.GetQueryableAsync();
+
+        return await AsyncExecuter.FirstAsync(query, book => book.Id == id);
+    }
+
+    protected override async Task<IQueryable<Book>?> CreateEntityQueryOrNullAsync(Guid id)
+    {
+        var query = await ReadOnlyRepository.GetQueryableAsync();
+
+        return query.Where(book => book.Id == id);
+    }
+
+    protected override IQueryable<Book> ApplyDefaultSorting(IQueryable<Book> query)
+    {
+        return query.OrderBy(book => book.Id);
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookAppService.cs

@@ -0,0 +1,13 @@
+using System;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookAppService : CrudAppService<Book, BookDto, Guid>
+{
+    public BookAppService(IRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookAsyncProjectionAppService.cs

@@ -0,0 +1,46 @@
+#nullable enable
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Volo.Abp.Application.Dtos;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookAsyncProjectionAppService : CrudAppService<Book, BookDto, Guid>
+{
+    public const string Marker = "-async";
+
+    private readonly IBookNameSuffixProvider _suffixProvider;
+
+    public BookAsyncProjectionAppService(
+        IRepository<Book, Guid> repository,
+        IBookNameSuffixProvider suffixProvider)
+        : base(repository)
+    {
+        _suffixProvider = suffixProvider;
+    }
+
+    protected override async Task<IQueryable<BookDto>?> CreateGetOutputDtoQueryOrNullAsync(Guid id)
+    {
+        var query = await Repository.GetQueryableAsync();
+
+        return await ProjectAsync(query.Where(book => book.Id == id));
+    }
+
+    protected override async Task<IQueryable<BookDto>?> CreateGetListOutputDtoQueryOrNullAsync(IQueryable<Book> query)
+    {
+        return await ProjectAsync(query);
+    }
+
+    private async Task<IQueryable<BookDto>> ProjectAsync(IQueryable<Book> query)
+    {
+        var suffix = await _suffixProvider.GetAsync();
+
+        return query.Select(book => new BookDto
+        {
+            Id = book.Id,
+            Name = book.Name + suffix
+        });
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookCustomizedAppService.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookCustomizedAppService : CrudAppService<Book, BookDto, Guid>
+{
+    public const string Marker = "-customized";
+
+    public BookCustomizedAppService(IRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+
+    protected override async Task<Book> GetEntityByIdAsync(Guid id)
+    {
+        var book = await base.GetEntityByIdAsync(id);
+        book.Name += Marker;
+        return book;
+    }
+
+    protected override Task<BookDto> MapToGetOutputDtoAsync(Book entity)
+    {
+        return Task.FromResult(new BookDto { Id = entity.Id, Name = entity.Name + Marker });
+    }
+
+    protected override Task<List<BookDto>> MapToGetListOutputDtosAsync(List<Book> entities)
+    {
+        return Task.FromResult(entities.ConvertAll(entity => new BookDto { Id = entity.Id, Name = entity.Name + Marker }));
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookDetailAppService.cs

@@ -0,0 +1,15 @@
+using System;
+using Volo.Abp.Application.Dtos;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookDetailAppService :
+    ReadOnlyAppService<Book, BookDetailDto, BookDto, Guid, PagedAndSortedResultRequestDto>
+{
+    public BookDetailAppService(IReadOnlyRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookDetailDto.cs

@@ -0,0 +1,9 @@
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookDetailDto : EntityDto<Guid>
+{
+    public string Name { get; set; } = default!;
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookDetailProjector.cs

@@ -0,0 +1,18 @@
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookDetailProjector : IQueryProjector<Book, BookDetailDto>
+{
+    public const string Marker = "-detail";
+
+    public IQueryable<BookDetailDto> ProjectTo(IQueryable<Book> source)
+    {
+        return source.Select(book => new BookDetailDto
+        {
+            Id = book.Id,
+            Name = book.Name + Marker
+        });
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookDto.cs

@@ -0,0 +1,9 @@
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookDto : EntityDto<Guid>
+{
+    public string Name { get; set; } = default!;
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookLiteAppService.cs

@@ -0,0 +1,13 @@
+using System;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookLiteAppService : CrudAppService<Book, BookLiteDto, Guid>
+{
+    public BookLiteAppService(IRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookLiteDto.cs

@@ -0,0 +1,9 @@
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookLiteDto : EntityDto<Guid>
+{
+    public string Name { get; set; } = default!;
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookObjectMapper.cs

@@ -0,0 +1,48 @@
+using Volo.Abp.DependencyInjection;
+using Volo.Abp.ObjectMapping;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookObjectMapper :
+    IObjectMapper<Book, BookDto>,
+    IObjectMapper<Book, BookLiteDto>,
+    IObjectMapper<BookDto, Book>,
+    ITransientDependency
+{
+    public const string Marker = "-mapped";
+
+    public BookDto Map(Book source)
+    {
+        return new BookDto { Id = source.Id, Name = source.Name + Marker };
+    }
+
+    public BookDto Map(Book source, BookDto destination)
+    {
+        destination.Id = source.Id;
+        destination.Name = source.Name + Marker;
+        return destination;
+    }
+
+    BookLiteDto IObjectMapper<Book, BookLiteDto>.Map(Book source)
+    {
+        return new BookLiteDto { Id = source.Id, Name = source.Name + Marker };
+    }
+
+    BookLiteDto IObjectMapper<Book, BookLiteDto>.Map(Book source, BookLiteDto destination)
+    {
+        destination.Id = source.Id;
+        destination.Name = source.Name + Marker;
+        return destination;
+    }
+
+    Book IObjectMapper<BookDto, Book>.Map(BookDto source)
+    {
+        return new Book(source.Id, source.Name, 0);
+    }
+
+    Book IObjectMapper<BookDto, Book>.Map(BookDto source, Book destination)
+    {
+        destination.Name = source.Name;
+        return destination;
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookPolicyCheckedAppService.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading.Tasks;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookPolicyCheckedException : Exception
+{
+
+}
+
+public class BookPolicyCheckedAppService : CrudAppService<Book, BookDto, Guid>
+{
+    public BookPolicyCheckedAppService(IRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+
+    protected override Task CheckGetPolicyAsync()
+    {
+        throw new BookPolicyCheckedException();
+    }
+
+    protected override Task CheckGetListPolicyAsync()
+    {
+        throw new BookPolicyCheckedException();
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookProjector.cs

@@ -0,0 +1,18 @@
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookProjector : IQueryProjector<Book, BookDto>
+{
+    public const string Marker = "-projected";
+
+    public IQueryable<BookDto> ProjectTo(IQueryable<Book> source)
+    {
+        return source.Select(book => new BookDto
+        {
+            Id = book.Id,
+            Name = book.Name + Marker
+        });
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookReadOnlyAppService.cs

@@ -0,0 +1,13 @@
+using System;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookReadOnlyAppService : ReadOnlyAppService<Book, BookDto, Guid>
+{
+    public BookReadOnlyAppService(IReadOnlyRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookRepository.cs

@@ -0,0 +1,106 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+using System.Threading;
+using System.Threading.Tasks;
+using Volo.Abp.DependencyInjection;
+using Volo.Abp.Domain.Entities;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+[ExposeServices(
+    typeof(IRepository<Book, Guid>),
+    typeof(IReadOnlyRepository<Book, Guid>),
+    typeof(IReadOnlyRepository<Book>))]
+public class BookRepository : RepositoryBase<Book, Guid>, ISingletonDependency
+{
+    private readonly List<Book> _books = new();
+
+    public BookRepository()
+        : base("InMemory")
+    {
+
+    }
+
+    public override Task<IQueryable<Book>> GetQueryableAsync()
+    {
+        return Task.FromResult(_books.AsQueryable());
+    }
+
+    [Obsolete("Use GetQueryableAsync method.")]
+    protected override IQueryable<Book> GetQueryable()
+    {
+        return _books.AsQueryable();
+    }
+
+    public override Task<Book> GetAsync(Guid id, bool includeDetails = true, CancellationToken cancellationToken = default)
+    {
+        var book = _books.FirstOrDefault(x => x.Id == id);
+        if (book == null)
+        {
+            throw new EntityNotFoundException(typeof(Book), id);
+        }
+
+        return Task.FromResult(book);
+    }
+
+    public override Task<Book> FindAsync(Guid id, bool includeDetails = true, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(_books.FirstOrDefault(x => x.Id == id));
+    }
+
+    public override Task<Book> FindAsync(Expression<Func<Book, bool>> predicate, bool includeDetails = true, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(_books.AsQueryable().FirstOrDefault(predicate));
+    }
+
+    public override Task<List<Book>> GetListAsync(bool includeDetails = false, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(_books.ToList());
+    }
+
+    public override Task<List<Book>> GetListAsync(Expression<Func<Book, bool>> predicate, bool includeDetails = false, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(_books.AsQueryable().Where(predicate).ToList());
+    }
+
+    public override Task<List<Book>> GetPagedListAsync(int skipCount, int maxResultCount, string sorting, bool includeDetails = false, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(_books.Skip(skipCount).Take(maxResultCount).ToList());
+    }
+
+    public override Task<long> GetCountAsync(CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult((long)_books.Count);
+    }
+
+    public override Task<Book> InsertAsync(Book entity, bool autoSave = false, CancellationToken cancellationToken = default)
+    {
+        _books.Add(entity);
+        return Task.FromResult(entity);
+    }
+
+    public override Task<Book> UpdateAsync(Book entity, bool autoSave = false, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(entity);
+    }
+
+    public override Task DeleteAsync(Book entity, bool autoSave = false, CancellationToken cancellationToken = default)
+    {
+        _books.Remove(entity);
+        return Task.CompletedTask;
+    }
+
+    public override Task DeleteAsync(Expression<Func<Book, bool>> predicate, bool autoSave = false, CancellationToken cancellationToken = default)
+    {
+        _books.RemoveAll(new Predicate<Book>(predicate.Compile()));
+        return Task.CompletedTask;
+    }
+
+    public override Task DeleteDirectAsync(Expression<Func<Book, bool>> predicate, CancellationToken cancellationToken = default)
+    {
+        return DeleteAsync(predicate);
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookStructAppService.cs

@@ -0,0 +1,13 @@
+using System;
+using Volo.Abp.Domain.Repositories;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookStructAppService : ReadOnlyAppService<Book, BookStructDto, Guid>
+{
+    public BookStructAppService(IReadOnlyRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookStructDto.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public struct BookStructDto
+{
+    public Guid Id { get; set; }
+
+    public string Name { get; set; }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookStructProjector.cs

@@ -0,0 +1,16 @@
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookStructProjector : IQueryProjector<Book, BookStructDto>
+{
+    public IQueryable<BookStructDto> ProjectTo(IQueryable<Book> source)
+    {
+        return source.Select(book => new BookStructDto
+        {
+            Id = book.Id,
+            Name = book.Name
+        });
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/BookWithoutProjectionAppService.cs

@@ -0,0 +1,33 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.ObjectMapping;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class BookWithoutProjectionAppService : CrudAppService<Book, BookDto, Guid>
+{
+    public const string Marker = "-not-projected";
+
+    protected override IQueryProjector<Book, BookDto>? GetOutputDtoQueryProjector => null;
+
+    protected override IQueryProjector<Book, BookDto>? GetListOutputDtoQueryProjector => null;
+
+    public BookWithoutProjectionAppService(IRepository<Book, Guid> repository)
+        : base(repository)
+    {
+
+    }
+
+    protected override Task<BookDto> MapToGetOutputDtoAsync(Book entity)
+    {
+        return Task.FromResult(new BookDto { Id = entity.Id, Name = entity.Name + Marker });
+    }
+
+    protected override Task<List<BookDto>> MapToGetListOutputDtosAsync(List<Book> entities)
+    {
+        return Task.FromResult(entities.ConvertAll(entity => new BookDto { Id = entity.Id, Name = entity.Name + Marker }));
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/IBookNameSuffixProvider.cs

@@ -0,0 +1,19 @@
+using System.Threading.Tasks;
+using Volo.Abp.DependencyInjection;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public interface IBookNameSuffixProvider
+{
+    Task<string> GetAsync();
+}
+
+public class BookNameSuffixProvider : IBookNameSuffixProvider, ITransientDependency
+{
+    public async Task<string> GetAsync()
+    {
+        await Task.Yield();
+
+        return BookAsyncProjectionAppService.Marker;
+    }
+}

framework/test/Volo.Abp.Ddd.Application.Tests/Volo/Abp/Application/Services/QueryProjection/QueryProjection_Tests.cs

@@ -0,0 +1,196 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using Shouldly;
+using Volo.Abp.Application.Dtos;
+using Volo.Abp.Domain.Entities;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.ObjectMapping;
+using Xunit;
+
+namespace Volo.Abp.Application.Services.QueryProjection;
+
+public class QueryProjection_Tests : AbpDddApplicationTestBase
+{
+    private readonly Guid _bookId = Guid.NewGuid();
+
+    public QueryProjection_Tests()
+    {
+        var repository = GetRequiredService<IRepository<Book, Guid>>();
+        repository.InsertAsync(new Book(_bookId, "Hitchhiker's Guide", 42)).GetAwaiter().GetResult();
+    }
+
+    [Fact]
+    public void Should_Resolve_The_Projector_Independent_From_The_Class_Name()
+    {
+        ServiceProvider.GetService<IQueryProjector<Book, BookDto>>()
+            .ShouldBeOfType<BookProjector>();
+    }
+
+    [Fact]
+    public async Task Should_Project_On_The_Query_For_CrudAppService()
+    {
+        var appService = GetRequiredService<BookAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookProjector.Marker);
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookProjector.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Project_On_The_Query_For_ReadOnlyAppService()
+    {
+        var appService = GetRequiredService<BookReadOnlyAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookProjector.Marker);
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookProjector.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Throw_EntityNotFoundException_If_The_Projected_Entity_Does_Not_Exist()
+    {
+        var appService = GetRequiredService<BookAppService>();
+
+        await Should.ThrowAsync<EntityNotFoundException>(async () => await appService.GetAsync(Guid.NewGuid()));
+    }
+
+    [Fact]
+    public async Task Should_Use_The_Object_Mapper_If_No_Projector_Was_Registered()
+    {
+        var appService = GetRequiredService<BookLiteAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookObjectMapper.Marker);
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookObjectMapper.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Use_The_Entity_Based_Overrides_If_The_Projection_Was_Disabled()
+    {
+        var appService = GetRequiredService<BookWithoutProjectionAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookWithoutProjectionAppService.Marker);
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookWithoutProjectionAppService.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Not_Use_The_Entity_Based_Overrides_While_Projecting()
+    {
+        var appService = GetRequiredService<BookCustomizedAppService>();
+
+        var dto = await appService.GetAsync(_bookId);
+
+        dto.Name.ShouldEndWith(BookProjector.Marker);
+        dto.Name.ShouldNotContain(BookCustomizedAppService.Marker);
+
+        var items = (await appService.GetListAsync(new PagedAndSortedResultRequestDto())).Items;
+
+        items[0].Name.ShouldEndWith(BookProjector.Marker);
+        items[0].Name.ShouldNotContain(BookCustomizedAppService.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Use_The_Object_Mapper_If_The_Entity_Query_Can_Not_Be_Created()
+    {
+        var appService = GetRequiredService<BookAbstractKeyAppService>();
+
+        //GetAsync has no query to project, it falls back to GetEntityByIdAsync
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookObjectMapper.Marker);
+
+        //GetListAsync always has a query, so it is still projected
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookProjector.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Use_The_Asynchronously_Created_Projection_Query()
+    {
+        var appService = GetRequiredService<BookAsyncProjectionAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookAsyncProjectionAppService.Marker);
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookAsyncProjectionAppService.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Throw_EntityNotFoundException_From_An_Overridden_Projection_Query()
+    {
+        var appService = GetRequiredService<BookAsyncProjectionAppService>();
+
+        await Should.ThrowAsync<EntityNotFoundException>(async () => await appService.GetAsync(Guid.NewGuid()));
+    }
+
+    [Fact]
+    public async Task Should_Throw_EntityNotFoundException_For_A_Value_Type_Dto()
+    {
+        var appService = GetRequiredService<BookStructAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldBe("Hitchhiker's Guide");
+
+        await Should.ThrowAsync<EntityNotFoundException>(async () => await appService.GetAsync(Guid.NewGuid()));
+    }
+
+    [Fact]
+    public async Task Should_Not_Project_On_The_Create_And_Update_Paths()
+    {
+        var appService = GetRequiredService<BookAppService>();
+
+        var created = await appService.CreateAsync(new BookDto { Name = "New Book" });
+        created.Name.ShouldEndWith(BookObjectMapper.Marker);
+
+        var updated = await appService.UpdateAsync(created.Id, new BookDto { Name = "Updated Book" });
+        updated.Name.ShouldEndWith(BookObjectMapper.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Use_A_Different_Projector_For_The_Get_And_The_List()
+    {
+        var appService = GetRequiredService<BookDetailAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookDetailProjector.Marker);
+        (await appService.GetListAsync(new PagedAndSortedResultRequestDto()))
+            .Items[0].Name.ShouldEndWith(BookProjector.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Apply_Paging_And_Sorting_Before_Projecting()
+    {
+        var repository = GetRequiredService<IRepository<Book, Guid>>();
+        await repository.InsertAsync(new Book(Guid.NewGuid(), "A Book", 1));
+        await repository.InsertAsync(new Book(Guid.NewGuid(), "Z Book", 2));
+
+        var appService = GetRequiredService<BookAppService>();
+
+        var firstPage = await appService.GetListAsync(
+            new PagedAndSortedResultRequestDto { MaxResultCount = 1, Sorting = "Name" });
+
+        firstPage.TotalCount.ShouldBe(3);
+        firstPage.Items.Count.ShouldBe(1);
+        firstPage.Items[0].Name.ShouldBe("A Book" + BookProjector.Marker);
+
+        var secondPage = await appService.GetListAsync(
+            new PagedAndSortedResultRequestDto { MaxResultCount = 1, SkipCount = 1, Sorting = "Name" });
+
+        secondPage.Items[0].Name.ShouldBe("Hitchhiker's Guide" + BookProjector.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Project_A_Single_Entity_From_An_AbstractKey_Application_Service()
+    {
+        var appService = GetRequiredService<BookAbstractKeyProjectingAppService>();
+
+        (await appService.GetAsync(_bookId)).Name.ShouldEndWith(BookProjector.Marker);
+    }
+
+    [Fact]
+    public async Task Should_Check_The_Policies_While_Projecting()
+    {
+        var appService = GetRequiredService<BookPolicyCheckedAppService>();
+
+        await Should.ThrowAsync<BookPolicyCheckedException>(async () => await appService.GetAsync(_bookId));
+        await Should.ThrowAsync<BookPolicyCheckedException>(async () =>
+            await appService.GetListAsync(new PagedAndSortedResultRequestDto()));
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/AbpEntityFrameworkCoreTestModule.cs

@@ -7,6 +7,7 @@ using Microsoft.Extensions.DependencyInjection;
 using Volo.Abp.Autofac;
 using Volo.Abp.Data;
 using Volo.Abp.Domain.Repositories;
+using Volo.Abp.EntityFrameworkCore.Applications;
 using Volo.Abp.EntityFrameworkCore.Domain;
 using Volo.Abp.EntityFrameworkCore.Sqlite;
 using Volo.Abp.EntityFrameworkCore.TestApp.FifthContext;
@@ -92,6 +93,7 @@ public class AbpEntityFrameworkCoreTestModule : AbpModule
             options.Configure(abpDbContextConfigurationContext =>
             {
                 abpDbContextConfigurationContext.UseSqlite().AddAbpDbContextOptionsExtension();
+                abpDbContextConfigurationContext.DbContextOptions.AddInterceptors(new SqlCommandCapture());
             });
         });
     }

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/EntityWithIntPkProjectionAppService.cs

@@ -0,0 +1,14 @@
+using Volo.Abp.Application.Services;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class EntityWithIntPkProjectionAppService : ReadOnlyAppService<EntityWithIntPk, EntityWithIntPkProjectionDto, int>
+{
+    public EntityWithIntPkProjectionAppService(IReadOnlyRepository<EntityWithIntPk, int> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/EntityWithIntPkProjectionDto.cs

@@ -0,0 +1,8 @@
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class EntityWithIntPkProjectionDto : EntityDto<int>
+{
+    public string Name { get; set; }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/EntityWithIntPkProjector.cs

@@ -0,0 +1,17 @@
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class EntityWithIntPkProjector : IQueryProjector<EntityWithIntPk, EntityWithIntPkProjectionDto>
+{
+    public IQueryable<EntityWithIntPkProjectionDto> ProjectTo(IQueryable<EntityWithIntPk> source)
+    {
+        return source.Select(entity => new EntityWithIntPkProjectionDto
+        {
+            Id = entity.Id,
+            Name = entity.Name
+        });
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/PersonProjectionAppService.cs

@@ -0,0 +1,15 @@
+using System;
+using Volo.Abp.Application.Services;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class PersonProjectionAppService : ReadOnlyAppService<Person, PersonProjectionDto, Guid>
+{
+    public PersonProjectionAppService(IReadOnlyRepository<Person, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/PersonProjectionDto.cs

@@ -0,0 +1,9 @@
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class PersonProjectionDto : EntityDto<Guid>
+{
+    public string Name { get; set; }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/PersonProjector.cs

@@ -0,0 +1,17 @@
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class PersonProjector : IQueryProjector<Person, PersonProjectionDto>
+{
+    public IQueryable<PersonProjectionDto> ProjectTo(IQueryable<Person> source)
+    {
+        return source.Select(person => new PersonProjectionDto
+        {
+            Id = person.Id,
+            Name = person.Name
+        });
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/PersonWithCityAppService.cs

@@ -0,0 +1,51 @@
+#nullable enable
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Volo.Abp.Application.Services;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+//City is another aggregate root, so Person has no City navigation property to project.
+public class PersonWithCityAppService : ReadOnlyAppService<Person, PersonWithCityDto, Guid>
+{
+    private readonly IReadOnlyRepository<City, Guid> _cityRepository;
+
+    public PersonWithCityAppService(
+        IReadOnlyRepository<Person, Guid> repository,
+        IReadOnlyRepository<City, Guid> cityRepository)
+        : base(repository)
+    {
+        _cityRepository = cityRepository;
+    }
+
+    protected override async Task<IQueryable<PersonWithCityDto>?> CreateGetOutputDtoQueryOrNullAsync(Guid id)
+    {
+        var people = await CreateEntityQueryOrNullAsync(id);
+
+        return people == null ? null : await JoinCitiesAsync(people);
+    }
+
+    protected override async Task<IQueryable<PersonWithCityDto>?> CreateGetListOutputDtoQueryOrNullAsync(IQueryable<Person> query)
+    {
+        return await JoinCitiesAsync(query);
+    }
+
+    //left join, an inner join would drop the people without a city and break the total count
+    private async Task<IQueryable<PersonWithCityDto>> JoinCitiesAsync(IQueryable<Person> people)
+    {
+        var cities = await _cityRepository.GetQueryableAsync();
+
+        return from person in people
+               join city in cities on person.CityId equals city.Id into personCities
+               from personCity in personCities.DefaultIfEmpty()
+               select new PersonWithCityDto
+               {
+                   Id = person.Id,
+                   Name = person.Name,
+                   CityName = personCity != null ? personCity.Name : null
+               };
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/PersonWithCityDto.cs

@@ -0,0 +1,12 @@
+#nullable enable
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class PersonWithCityDto : EntityDto<Guid>
+{
+    public string Name { get; set; }
+
+    public string? CityName { get; set; }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/QueryProjection_Tests.cs

@@ -0,0 +1,197 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.EntityFrameworkCore;
+using Shouldly;
+using Volo.Abp.Application.Dtos;
+using Volo.Abp.Domain.Entities;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.ObjectMapping;
+using Volo.Abp.TestApp;
+using Volo.Abp.TestApp.Application;
+using Volo.Abp.Threading;
+using Volo.Abp.TestApp.Domain;
+using Xunit;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class QueryProjection_Tests : EntityFrameworkCoreTestBase
+{
+    private readonly PersonProjectionAppService _personProjectionAppService;
+
+    public QueryProjection_Tests()
+    {
+        _personProjectionAppService = GetRequiredService<PersonProjectionAppService>();
+    }
+
+    [Fact]
+    public async Task Should_Get_A_Projected_Entity()
+    {
+        var dto = await _personProjectionAppService.GetAsync(TestDataBuilder.UserDouglasId);
+
+        dto.Id.ShouldBe(TestDataBuilder.UserDouglasId);
+        dto.Name.ShouldBe("Douglas");
+    }
+
+    [Fact]
+    public async Task Should_Throw_EntityNotFoundException_For_A_Missing_Entity()
+    {
+        (await Should.ThrowAsync<EntityNotFoundException<Person>>(
+            async () => await _personProjectionAppService.GetAsync(Guid.NewGuid()))
+        ).EntityType.ShouldBe(typeof(Person));
+    }
+
+    [Fact]
+    public async Task Should_Get_A_Projected_List()
+    {
+        var result = await _personProjectionAppService.GetListAsync(new PagedAndSortedResultRequestDto());
+
+        result.TotalCount.ShouldBeGreaterThan(0);
+        result.Items.Count.ShouldBe((int)result.TotalCount);
+        result.Items.ShouldContain(x => x.Name == "Douglas");
+    }
+
+    [Fact]
+    public async Task Should_Get_A_Projected_Entity_With_A_Non_Guid_Key()
+    {
+        var appService = GetRequiredService<EntityWithIntPkProjectionAppService>();
+        var entity = await WithUnitOfWorkAsync(
+            async () => await GetRequiredService<IRepository<EntityWithIntPk, int>>().FirstAsync());
+
+        (await appService.GetAsync(entity.Id)).Name.ShouldBe(entity.Name);
+    }
+
+    [Fact]
+    public async Task Should_Apply_The_Soft_Delete_Filter_While_Projecting()
+    {
+        var result = await _personProjectionAppService.GetListAsync(new PagedAndSortedResultRequestDto());
+
+        result.Items.ShouldNotContain(x => x.Id == TestDataBuilder.UserJohnDeletedId);
+    }
+
+    [Fact]
+    public async Task Should_Only_Select_The_Projected_Columns()
+    {
+        await WithUnitOfWorkAsync(async () =>
+        {
+            var repository = GetRequiredService<IReadOnlyRepository<Person, Guid>>();
+            var projector = GetRequiredService<IQueryProjector<Person, PersonProjectionDto>>();
+
+            var sql = projector.ProjectTo(await repository.GetQueryableAsync()).ToQueryString();
+
+            sql.ShouldContain("\"Name\"");
+            sql.ShouldNotContain("\"Birthday\"");
+            sql.ShouldNotContain("\"ExtraProperties\"");
+
+            //the data filters are still a part of the query
+            sql.ShouldContain("Is_Deleted");
+        });
+    }
+
+    [Fact]
+    public async Task Should_Join_Another_Aggregate_While_Projecting()
+    {
+        var appService = GetRequiredService<PersonWithCityAppService>();
+
+        var dto = await appService.GetAsync(TestDataBuilder.UserDouglasId);
+        dto.CityName.ShouldBe("London");
+
+        var result = await appService.GetListAsync(new PagedAndSortedResultRequestDto());
+        result.Items.ShouldContain(x => x.CityName == "London");
+    }
+
+    [Fact]
+    public async Task Should_Keep_The_Total_Count_While_Joining_Another_Aggregate()
+    {
+        await WithUnitOfWorkAsync(async () =>
+        {
+            await GetRequiredService<IRepository<Person, Guid>>()
+                .InsertAsync(new Person(Guid.NewGuid(), "PersonWithoutCity", 30), autoSave: true);
+        });
+
+        var result = await GetRequiredService<PersonWithCityAppService>()
+            .GetListAsync(new PagedAndSortedResultRequestDto());
+
+        result.Items.Count.ShouldBe((int)result.TotalCount);
+        result.Items.ShouldContain(x => x.Name == "PersonWithoutCity" && x.CityName == null);
+    }
+
+    [Fact]
+    public async Task Should_Execute_The_Projected_Query_From_The_Application_Service()
+    {
+        System.Collections.Concurrent.ConcurrentQueue<string> commands;
+        using (SqlCommandCapture.Begin(out commands))
+        {
+            await GetRequiredService<PersonProjectionAppService>()
+                .GetListAsync(new PagedAndSortedResultRequestDto());
+        }
+
+        //the application service must run the projection itself, not materialize the entities first
+        var selects = commands.Where(x => x.Contains("FROM \"People\"") && !x.Contains("COUNT")).ToList();
+
+        var select = selects.ShouldHaveSingleItem();
+        select.ShouldContain("\"Name\"");
+        select.ShouldNotContain("\"Birthday\"");
+        select.ShouldNotContain("\"ExtraProperties\"");
+    }
+
+    [Fact]
+    public async Task Should_Apply_The_Multi_Tenancy_Filter_While_Projecting()
+    {
+        var result = await GetRequiredService<PersonProjectionAppService>()
+            .GetListAsync(new PagedAndSortedResultRequestDto());
+
+        result.Items.ShouldNotContain(x => x.Name.StartsWith(TestDataBuilder.TenantId1.ToString()));
+    }
+
+    [Fact]
+    public async Task Should_Use_The_Ambient_Cancellation_Token_While_Projecting()
+    {
+        var cancellationTokenProvider = GetRequiredService<ICancellationTokenProvider>();
+        var appService = GetRequiredService<PersonProjectionAppService>();
+
+        //the entity path passes it through Repository.GetAsync, so the projected path must not regress
+        using (cancellationTokenProvider.Use(new CancellationToken(canceled: true)))
+        {
+            await Should.ThrowAsync<OperationCanceledException>(async () =>
+                await appService.GetAsync(TestDataBuilder.UserDouglasId));
+        }
+    }
+
+    [Fact]
+    public async Task Should_Execute_The_Projected_Query_From_The_Application_Service_For_A_Single_Entity()
+    {
+        System.Collections.Concurrent.ConcurrentQueue<string> commands;
+        using (SqlCommandCapture.Begin(out commands))
+        {
+            await GetRequiredService<PersonProjectionAppService>().GetAsync(TestDataBuilder.UserDouglasId);
+        }
+
+        var selects = commands.Where(x => x.Contains("FROM \"People\"")).ToList();
+
+        var select = selects.ShouldHaveSingleItem();
+        select.ShouldContain("\"Name\"");
+        select.ShouldNotContain("\"Birthday\"");
+        select.ShouldNotContain("\"ExtraProperties\"");
+    }
+
+    [Fact]
+    public async Task Should_Apply_The_Data_Filters_Of_The_Joined_Aggregate()
+    {
+        var cityRepository = GetRequiredService<IRepository<City, Guid>>();
+
+        await WithUnitOfWorkAsync(async () =>
+        {
+            var london = await cityRepository.GetAsync(TestDataBuilder.LondonCityId);
+            await cityRepository.DeleteAsync(london, autoSave: true);
+        });
+
+        var result = await GetRequiredService<PersonWithCityAppService>()
+            .GetListAsync(new PagedAndSortedResultRequestDto());
+
+        //the soft deleted city is filtered out, the left join still keeps the person
+        result.Items.ShouldContain(x => x.Name == "Douglas" && x.CityName == null);
+    }
+}

framework/test/Volo.Abp.EntityFrameworkCore.Tests/Volo/Abp/EntityFrameworkCore/Applications/SqlCommandCapture.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Data.Common;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.EntityFrameworkCore.Diagnostics;
+
+namespace Volo.Abp.EntityFrameworkCore.Applications;
+
+public class SqlCommandCapture : DbCommandInterceptor
+{
+    private static readonly AsyncLocal<ConcurrentQueue<string>> Commands = new();
+
+    public static IDisposable Begin(out ConcurrentQueue<string> commands)
+    {
+        commands = new ConcurrentQueue<string>();
+        Commands.Value = commands;
+        return new DisposeAction(() => Commands.Value = null);
+    }
+
+    public override InterceptionResult<DbDataReader> ReaderExecuting(
+        DbCommand command,
+        CommandEventData eventData,
+        InterceptionResult<DbDataReader> result)
+    {
+        Commands.Value?.Enqueue(command.CommandText);
+        return base.ReaderExecuting(command, eventData, result);
+    }
+
+    public override ValueTask<InterceptionResult<DbDataReader>> ReaderExecutingAsync(
+        DbCommand command,
+        CommandEventData eventData,
+        InterceptionResult<DbDataReader> result,
+        CancellationToken cancellationToken = default)
+    {
+        Commands.Value?.Enqueue(command.CommandText);
+        return base.ReaderExecutingAsync(command, eventData, result, cancellationToken);
+    }
+}

framework/test/Volo.Abp.Mapperly.Tests/Volo/Abp/Mapperly/AbpMapperlyQueryProjection_Tests.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.Extensions.DependencyInjection;
+using Shouldly;
+using Volo.Abp.Mapperly.SampleClasses;
+using Volo.Abp.ObjectMapping;
+using Volo.Abp.Testing;
+using Xunit;
+
+namespace Volo.Abp.Mapperly;
+
+public class AbpMapperlyQueryProjection_Tests : AbpIntegratedTest<MapperlyTestModule>
+{
+    [Fact]
+    public void Should_Project_A_Queryable()
+    {
+        var queryProjector = ServiceProvider.GetRequiredService<IQueryProjector<MyEntity, MyEntityDto>>();
+
+        var entities = new List<MyEntity>
+        {
+            new MyEntity { Id = Guid.NewGuid(), Number = 42 }
+        }.AsQueryable();
+
+        var dtos = queryProjector.ProjectTo(entities).ToList();
+
+        dtos.Count.ShouldBe(1);
+        dtos[0].Id.ShouldBe(entities.First().Id);
+        dtos[0].Number.ShouldBe(42);
+    }
+
+
+    [Fact]
+    public void Should_Register_A_Projector_Only_Once()
+    {
+        //MyEntityQueryProjector also matches the class name convention of ExposeServicesAttribute
+        ServiceProvider.GetServices<IQueryProjector<MyEntity, MyEntityDto>>().ShouldHaveSingleItem();
+    }
+}

framework/test/Volo.Abp.Mapperly.Tests/Volo/Abp/Mapperly/SampleClasses/MyEntityQueryProjector.cs

@@ -0,0 +1,11 @@
+using System.Linq;
+using Riok.Mapperly.Abstractions;
+using Volo.Abp.ObjectMapping;
+
+namespace Volo.Abp.Mapperly.SampleClasses;
+
+[Mapper(RequiredMappingStrategy = RequiredMappingStrategy.Target)]
+public partial class MyEntityQueryProjector : IQueryProjector<MyEntity, MyEntityDto>
+{
+    public partial IQueryable<MyEntityDto> ProjectTo(IQueryable<MyEntity> source);
+}

framework/test/Volo.Abp.MongoDB.Tests/Volo/Abp/MongoDB/Applications/PersonProjectionAppService.cs

@@ -0,0 +1,15 @@
+using System;
+using Volo.Abp.Application.Services;
+using Volo.Abp.Domain.Repositories;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.MongoDB.Applications;
+
+public class PersonProjectionAppService : ReadOnlyAppService<Person, PersonProjectionDto, Guid>
+{
+    public PersonProjectionAppService(IReadOnlyRepository<Person, Guid> repository)
+        : base(repository)
+    {
+
+    }
+}

framework/test/Volo.Abp.MongoDB.Tests/Volo/Abp/MongoDB/Applications/PersonProjectionDto.cs

@@ -0,0 +1,9 @@
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Volo.Abp.MongoDB.Applications;
+
+public class PersonProjectionDto : EntityDto<Guid>
+{
+    public string Name { get; set; }
+}

framework/test/Volo.Abp.MongoDB.Tests/Volo/Abp/MongoDB/Applications/PersonProjector.cs

@@ -0,0 +1,17 @@
+using System.Linq;
+using Volo.Abp.ObjectMapping;
+using Volo.Abp.TestApp.Domain;
+
+namespace Volo.Abp.MongoDB.Applications;
+
+public class PersonProjector : IQueryProjector<Person, PersonProjectionDto>
+{
+    public IQueryable<PersonProjectionDto> ProjectTo(IQueryable<Person> source)
+    {
+        return source.Select(person => new PersonProjectionDto
+        {
+            Id = person.Id,
+            Name = person.Name
+        });
+    }
+}

framework/test/Volo.Abp.MongoDB.Tests/Volo/Abp/MongoDB/Applications/QueryProjection_Tests.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Threading.Tasks;
+using Shouldly;
+using Volo.Abp.Application.Dtos;
+using Volo.Abp.Domain.Entities;
+using Volo.Abp.TestApp;
+using Volo.Abp.TestApp.Domain;
+using Xunit;
+
+namespace Volo.Abp.MongoDB.Applications;
+
+[Collection(MongoTestCollection.Name)]
+public class QueryProjection_Tests : MongoDbTestBase
+{
+    private readonly PersonProjectionAppService _personProjectionAppService;
+
+    public QueryProjection_Tests()
+    {
+        _personProjectionAppService = GetRequiredService<PersonProjectionAppService>();
+    }
+
+    [Fact]
+    public async Task Should_Get_A_Projected_Entity()
+    {
+        var dto = await _personProjectionAppService.GetAsync(TestDataBuilder.UserDouglasId);
+
+        dto.Id.ShouldBe(TestDataBuilder.UserDouglasId);
+        dto.Name.ShouldBe("Douglas");
+    }
+
+    [Fact]
+    public async Task Should_Throw_EntityNotFoundException_For_A_Missing_Entity()
+    {
+        await Should.ThrowAsync<EntityNotFoundException<Person>>(
+            async () => await _personProjectionAppService.GetAsync(Guid.NewGuid()));
+    }
+
+    [Fact]
+    public async Task Should_Get_A_Projected_List()
+    {
+        var result = await _personProjectionAppService.GetListAsync(new PagedAndSortedResultRequestDto());
+
+        result.TotalCount.ShouldBeGreaterThan(0);
+        result.Items.ShouldContain(x => x.Name == "Douglas");
+    }
+}