Create post for "Many-to-many relationship" article

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

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+# Many to Many Relationship with ABP and EF Core
+
+## Introduction 
+
+In this article, we'll create a **BookStore** application like in [the ABP tutorial](https://docs.abp.io/en/abp/latest/Tutorials/Part-1?UI=MVC&DB=EF) and add an extra `Category` feature to demonstrate how we can manage the many-to-many relationship with ABP-based applications (by following DDD rules).
+
+You can see the ER Diagram of our application under below. This diagram will be helpful for us to demonstrate the relations between our entities.
+
+![ER-Diagram](./er-diagram.png)
+
+When we've examined the ER Diagram, we can see the one-to-many relationship between **Author** and **Book** tables and also the many-to-many relationship (**BookCategory** table) between **Book** and **Category** tables. Cause, there can be more than one category on each book and vice-versa in our scenario.
+
+### Source Code
+
+You can find the source code of the application at https://github.com/EngincanV/ABP-Many-to-Many-Relationship-Demo .
+## 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):
+
+```bash
+abp new BookStore -t app --version 5.0.0-beta.2
+```
+
+* Our project boilerplate will be ready after the download is finished. Then, we can open the solution and starts to the development.
+
+## Starting the Development
+
+Let's start with creating our Domain Entities. 
+
+### Step 1 - (Creating the 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)
+
+Open the entity classes and add the following codes to each of these classes.
+
+* **Author.cs**
+
+```csharp
+using System;
+using JetBrains.Annotations;
+using Volo.Abp;
+using Volo.Abp.Domain.Entities.Auditing;
+
+namespace BookStore.Authors
+{
+    public class Author : FullAuditedAggregateRoot<Guid>
+    {
+        public string Name { get; private set; }
+        
+        public DateTime BirthDate { get; set; }
+        
+        public string ShortBio { get; set; }
+        
+        /* This constructor is for deserialization / ORM purpose */
+        private Author()
+        {
+        }
+
+        public Author(Guid id, [NotNull] string name, DateTime birthDate, [CanBeNull] string shortBio = null)
+            : base(id)
+        {
+            SetName(name);
+            BirthDate = birthDate;
+            ShortBio = shortBio;
+        }
+
+        public void SetName([NotNull] string name)
+        {
+            Name = Check.NotNullOrWhiteSpace(
+                name,
+                nameof(name),
+                maxLength: AuthorConsts.MaxNameLength
+            );
+        }
+    }
+}
+```
+
+> We'll create the `AuthorConsts` class later in this step.
+
+* **Book.cs**
+
+```csharp
+using System;
+using Volo.Abp.Domain.Entities.Auditing;
+
+namespace BookStore.Books
+{
+    public class Book : FullAuditedAggregateRoot<Guid>
+    {
+        public Guid AuthorId { get; set; }
+
+        public string Name { get; set; }
+
+        public BookType Type { get; set; }
+
+        public DateTime PublishDate { get; set; }
+
+        public float Price { get; set; }
+        
+        /* This constructor is for deserialization / ORM purpose */
+        private Book()
+        {
+        }
+
+        public Book(Guid id, Guid authorId, string name, BookType type, DateTime publishDate, float price) 
+            : base(id)
+        {
+            AuthorId = authorId;
+            Name = name;
+            Type = type;
+            PublishDate = publishDate;
+            Price = price;
+        }
+    }
+}
+```
+
+> We'll create the `BookType` enum later in this step.
+
+* **Category.cs**
+
+```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; }
+
+        public bool IsActive { get; set; }
+
+        /* This constructor is for deserialization / ORM purpose */
+        private Category()
+        {
+        }
+
+        public Category(Guid id, string name, bool isActive = true) : base(id)
+        {
+            SetName(name);
+            IsActive = isActive;
+        }
+
+        public Category SetName(string name)
+        { 
+            Name = Check.NotNullOrWhiteSpace(name, nameof(name), BookConsts.MaxNameLength);
+            return this;
+        }
+    }
+}
+```
+
+> We'll create the `BookConsts` class later in this step.
+
+* **BookCategory.cs**
+
+```csharp
+using System;
+using Volo.Abp.Domain.Entities;
+
+namespace BookStore.Books
+{
+    public class BookCategory : Entity
+    {
+        public Guid BookId { get; protected set; }
+
+        public Guid CategoryId { get; protected set; }
+
+        private BookCategory()
+        {
+        }
+
+        public BookCategory(Guid bookId, Guid categoryId)
+        {
+            BookId = bookId;
+            CategoryId = categoryId;
+        }
+        
+        public override object[] GetKeys()
+        {
+            return new object[] {BookId, CategoryId};
+        }
+    }
+}
+```
+
+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.
+
+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. 
+
+>  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)
+
+
+
+* After defining our entities, now we can open the `BookStore.Domain.Shared` project and add the relevant constants and enums from there.
+
+* 
+
+### Step 2 - (Database Integration)
+
+
+---
+
+* Create the Domain Entities ✓
+* Database Integration ✓
+* Db migration ✓
+* app services (category -> crud app service)
+* ui