Update the many-to-many relationship article

docs/en/Community-Articles/2021-10-31-Many-to-Many-Relationship-with-ABP-and-EF-Core/POST.md

@@ -13,11 +13,18 @@ When we've examined the ER Diagram, we can see the one-to-many relationship betw
 ### Source Code
 
 You can find the source code of the application at https://github.com/EngincanV/ABP-Many-to-Many-Relationship-Demo .
+
+### Screenshot of The Final Application
+
+At the end of this article, we will have created an application as in the below image.
+
+![Book Homepage](./demo.png)
+
 ## Creating the Solution
 
 In this article, I will create a new startup template with EF Core as a database provider and MVC for UI framework.
 
-* We can create a new startup template by using [ABP CLI](https://docs.abp.io/en/abp/latest/CLI):
+* We can create a new startup template by using the [ABP CLI](https://docs.abp.io/en/abp/latest/CLI):
 
 ```bash
 abp new BookStore -t app --version 5.0.0-beta.2
@@ -33,7 +40,7 @@ Let's start with creating our Domain Entities.
 
 We can create a folder-structure under the `BookStore.Domain` project like in the below image.
 
-![Domain-Layer-Folder-Structure](./domain-entity-structure.png)
+![Domain-Layer-Folder-Structure](./domain-file-structure.png)
 
 Open the entity classes and add the following codes to each of these classes.
 
@@ -86,6 +93,10 @@ namespace BookStore.Authors
 
 ```csharp
 using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+using Volo.Abp;
 using Volo.Abp.Domain.Entities.Auditing;
 
 namespace BookStore.Books
@@ -94,70 +105,94 @@ namespace BookStore.Books
     {
         public Guid AuthorId { get; set; }
 
-        public string Name { get; set; }
-
-        public BookType Type { get; set; }
+        public string Name { get; private set; }
 
         public DateTime PublishDate { get; set; }
 
         public float Price { get; set; }
-        
-        /* This constructor is for deserialization / ORM purpose */
+
+        public ICollection<BookCategory> Categories { get; private set; }
+
         private Book()
         {
         }
 
-        public Book(Guid id, Guid authorId, string name, BookType type, DateTime publishDate, float price) 
+        public Book(Guid id, Guid authorId, string name, DateTime publishDate, float price) 
             : base(id)
         {
             AuthorId = authorId;
-            Name = name;
-            Type = type;
+            SetName(name);
             PublishDate = publishDate;
             Price = price;
+
+            Categories = new Collection<BookCategory>();
         }
-    }
-}
-```
 
-> We'll create the `BookType` enum later in this step.
+        public void SetName(string name)
+        {
+            Name = Check.NotNullOrWhiteSpace(name, nameof(name), BookConsts.MaxNameLength);
+        }
 
-* **Category.cs**
+        public void AddCategory(Guid categoryId)
+        {
+            Check.NotNull(categoryId, nameof(categoryId));
+
+            if (IsInCategory(categoryId))
+            {
+                return;
+            }
+            
+            Categories.Add(new BookCategory(bookId: Id, categoryId));
+        }
 
-```csharp
-using System;
-using Volo.Abp;
-using Volo.Abp.Domain.Entities.Auditing;
+        public void RemoveCategory(Guid categoryId)
+        {
+            Check.NotNull(categoryId, nameof(categoryId));
 
-namespace BookStore.Categories
-{
-    public class Category : AuditedAggregateRoot<Guid>
-    {
-        public string Name { get; private set; }
+            if (!IsInCategory(categoryId))
+            {
+                return;
+            }
 
-        public bool IsActive { get; set; }
+            Categories.RemoveAll(x => x.CategoryId == categoryId);
+        }
 
-        /* This constructor is for deserialization / ORM purpose */
-        private Category()
+        public void RemoveAllCategoriesExceptGivenIds(List<Guid> categoryIds)
         {
+            Check.NotNullOrEmpty(categoryIds, nameof(categoryIds));
+            
+            Categories.RemoveAll(x => !categoryIds.Contains(x.CategoryId));
         }
 
-        public Category(Guid id, string name, bool isActive = true) : base(id)
+        public void RemoveAllCategories()
         {
-            SetName(name);
-            IsActive = isActive;
+            Categories.RemoveAll(x => x.BookId == Id);
         }
 
-        public Category SetName(string name)
-        { 
-            Name = Check.NotNullOrWhiteSpace(name, nameof(name), BookConsts.MaxNameLength);
-            return this;
+        private bool IsInCategory(Guid categoryId)
+        {
+            return Categories.Any(x => x.CategoryId == categoryId);
         }
     }
 }
 ```
 
-> We'll create the `BookConsts` class later in this step.
+* In our scenerio, a book can have more than one category and a category can have more 
+than one book so it's obvious to we need to establish many-to-many relationship between them.
+
+* For achieve this we will create an **join entity** named `BookCategory`, and this class
+will simply have variables named `BookId` and `CategoryId`.
+
+* To manage this **join entity**, we can add it as sub-collection to the **Book** entity, as we do above. We add this sub-collection
+to **Book** class instead of **Category** class, because a book can have tens or mostly hundreds of category in it but in other
+view a category can have more than hundred (or even way much) book inside of it.
+
+* It is a significant performance problem to load thousands of items whenever you query a category. Therefore it's make much sense to add that sub-collection to `Book` entity. (Don't forget: **An aggregate (with the root entity and sub-collections) should
+be serializable and transferrable on the wire as a single unit.**)
+
+* Notice that, `BookCategory` is not an **Aggregate Root** so we are not violating the one of the base rule about Aggregate Root (Rule: "Reference Other Aggregates Only by ID").
+
+> We'll create the `BookCategory` and `BookConsts` classes later in this step.
 
 * **BookCategory.cs**
 
@@ -173,6 +208,7 @@ namespace BookStore.Books
 
         public Guid CategoryId { get; protected set; }
 
+        /* This constructor is for deserialization / ORM purpose */
         private BookCategory()
         {
         }
@@ -191,27 +227,52 @@ namespace BookStore.Books
 }
 ```
 
-Here, as you can notice we've defined the `BookCategory` as the **Join Table** for our many-to-many relationship and ensure the required properties (BookId and CategoryId) must be set to create this object.
+* Here, as you can notice we've defined the `BookCategory` as the **Join Table/Entity** for our many-to-many relationship and ensure the required properties (BookId and CategoryId) must be set to create this object.
 
-And also we've derived this class from `Entity` class and therefore we've had to override the **GetKeys** method of this class to define **Composite Key**
+* And also we've derived this class from `Entity` class and therefore we've had to override the **GetKeys** method of this class to define **Composite Key**.
 
-> The composite key is composed of `BookId` and `CategoryId` in our case. 
+> The composite key is composed of `BookId` and `CategoryId` in our case. And they are unique with together.
 
 >  For more information about **Entities with Composite Keys**, you can read the relavant section from [Entites documentation](https://docs.abp.io/en/abp/latest/Entities#entities-with-composite-keys)
 
+//TODO: add BookManager class later?
 
+* **Category.cs**
 
-* After defining our entities, now we can open the `BookStore.Domain.Shared` project and add the relevant constants and enums from there.
+```csharp
+using System;
+using Volo.Abp;
+using Volo.Abp.Domain.Entities.Auditing;
 
-* 
+namespace BookStore.Categories
+{
+    public class Category : AuditedAggregateRoot<Guid>
+    {
+        public string Name { get; private set; }
+
+        /* This constructor is for deserialization / ORM purpose */
+        private Category()
+        {
+        }
+
+        public Category(Guid id, string name) : base(id)
+        {
+            SetName(name);
+        }
+
+        public Category SetName(string name)
+        { 
+            Name = Check.NotNullOrWhiteSpace(name, nameof(name), CategoryConsts.MaxNameLength);
+            return this;
+        }
+    }
+}
+```
 
 ### Step 2 - (Database Integration)
 
+### Step 3 - (Database Migration)
 
----
+### Step 4 - (Create Application Services)
 
-* Create the Domain Entities ✓
-* Database Integration ✓
-* Db migration ✓
-* app services (category -> crud app service)
-* ui
+### Step 5 - (UI)