Documented data filtering.

docs/en/Data-Filtering.md

@@ -1,3 +1,122 @@
 # Data Filtering
 
-TODO
+[Volo.Abp.Data](https://www.nuget.org/packages/Volo.Abp.Data) package defines services to automatically filter data on querying from a database.
+
+## Pre-Defined Filters
+
+ABP defines some filters out of the box.
+
+### ISoftDelete
+
+Used to mark an entity as deleted instead of actually deleting it. Implement the `ISoftDelete` interface to make your entity "soft delete".
+
+Example:
+
+````csharp
+using System;
+using Volo.Abp;
+using Volo.Abp.Domain.Entities;
+
+namespace Acme.BookStore
+{
+    public class Book : AggregateRoot<Guid>, ISoftDelete
+    {
+        public string Name { get; set; }
+
+        public bool IsDeleted { get; set; } //Defined by ISoftDelete
+    }
+}
+````
+
+`ISoftDelete` defines the `IsDeleted` property. When you delete a book using [repositories](Repositories.md), ABP automatically sets `IsDeleted` to true and protects it from actual deletion (you can also manually set the `IsDeleted` property to true if you need). In addition, it **automatically filters deleted entities** when you query the database.
+
+> `ISoftDelete` filter is enabled by default and you can not get deleted entities from database unless you explicitly disable it. See the `IDataFilter` service below.
+
+### IMultiTenant
+
+[Multi-tenancy](Multi-Tenancy.md) is an efficient way of creating SaaS applications. Once you create a multi-tenant application, you typically want to isolate data between tenants. Implement `IMultiTenant` interface to make your entity "multi-tenant aware".
+
+Example:
+
+````csharp
+using System;
+using Volo.Abp;
+using Volo.Abp.Domain.Entities;
+using Volo.Abp.MultiTenancy;
+
+namespace Acme.BookStore
+{
+    public class Book : AggregateRoot<Guid>, ISoftDelete, IMultiTenant
+    {
+        public string Name { get; set; }
+
+        public bool IsDeleted { get; set; } //Defined by ISoftDelete
+
+        public Guid? TenantId { get; set; } //Defined by IMultiTenant
+    }
+}
+````
+
+`IMultiTenant` interface defines the `TenantId` property which is then used to automatically filter the entities for the current tenant. See the [Multi-tenancy](Multi-Tenancy.md) document for more.
+
+## IDataFilter Service: Enable/Disable Data Filters
+
+You can control the filters using `IDataFilter` service.
+
+Example:
+
+````csharp
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Volo.Abp;
+using Volo.Abp.Data;
+using Volo.Abp.DependencyInjection;
+using Volo.Abp.Domain.Repositories;
+
+namespace Acme.BookStore
+{
+    public class MyBookService : ITransientDependency
+    {
+        private readonly IDataFilter _dataFilter;
+        private readonly IRepository<Book, Guid> _bookRepository;
+
+        public MyBookService(
+            IDataFilter dataFilter,
+            IRepository<Book, Guid> bookRepository)
+        {
+            _dataFilter = dataFilter;
+            _bookRepository = bookRepository;
+        }
+
+        public async Task<List<Book>> GetAllBooksIncludingDeletedAsync()
+        {
+            //Temporary disable the ISoftDelete filter
+            using (_dataFilter.Disable<ISoftDelete>())
+            {
+                return await _bookRepository.GetListAsync();
+            }
+        }
+    }
+}
+````
+
+* [Inject](Dependency-Injection.md) the `IDataFilter` service to your class.
+* Use the `Disable` method within a `using` statement to create a code block where the `ISoftDelete` filter is disabled inside it (Always use it inside a `using` block to guarantee that the filter is reset to its previous state).
+
+`IDataFilter.Enable` method can be used to enable a filter. `Enable` and `Disable` methods can be used in a nested way to define inner scopes.
+
+## AbpDataFilterOptions
+
+`AbpDataFilterOptions` can be used to [set options](Options.md) for the data filter system.
+
+The example code below disables the `ISoftDelete` filter by default which will cause to include deleted entities when you query the database unless you explicitly enable the filter:
+
+````csharp
+Configure<AbpDataFilterOptions>(options =>
+{
+    options.DefaultStates[typeof(ISoftDelete)] = new DataFilterState(isEnabled: false);
+});
+````
+
+> Carefully change defaults for global filters, especially if you are using a pre-built module which might be developed assuming the soft delete filter is turned on by default. But you can do it for your own defined filters safely.