A many-to-many relationship can be represented using a junction table with two foreign key properties that reference the primary key properties of the related entities. Before EF Core 5.0, it was not possible to establish a many-to-many relationship in Entity Framework Core without a joining entity class and configuration via the Fluent API.
- In Entity Framework Core 5.0 and later, it has full support for many-to-many relationships by convention, without explicitly defining a joined entity.
- The convention is that, if you have two entities
AandBand you want to create a many-to-many relationship between them, you need to have two navigation properties in each entity, each with a collection of the other entity. - Many-to-many relationships require a collection navigation property on both sides.
Here's an example of how to create a many-to-many relationship between a Book and a Category entity by convention:
public class Author
{
public int AuthorId { get; set; }
public string? FirstName { get; set; }
public string? LastName { get; set; }
public ICollection<Book> Books { get; set; }
}
public class Book
{
public int BookId { get; set; }
public string? Title { get; set; }
public Author Author { get; set; }
public ICollection<Category> Categories { get; set; }
}
public class Category
{
public int CategoryId { get; set; }
public string? CategoryName { get; set; }
public ICollection<Book> Books { get; set; }
}
As you can see the Book class contains a collection of Categories, and the Category class contains a collection of Books. EF Core 5.0 and later recognizes this as a many-to-many relationship by convention and there is no need for configuration in OnModelCreating.
public class LibraryContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(@"Data Source=(localdb)\ProjectsV13;Initial Catalog=LibraryDb;");
}
public DbSet<Book> Books { get; set; }
public DbSet<Author> Authors { get; set; }
public DbSet<Category> Categories { get; set; }
}
When you create migration or call the EnsureCreatedmethod, it will create the following tables including the join table.
CREATE TABLE [dbo].[Books] (
[BookId] INT IDENTITY (1, 1) NOT NULL,
[Title] NVARCHAR (MAX) NULL,
[AuthorId] INT NOT NULL,
CONSTRAINT [PK_Books] PRIMARY KEY CLUSTERED ([BookId] ASC),
CONSTRAINT [FK_Books_Authors_AuthorId] FOREIGN KEY ([AuthorId]) REFERENCES [dbo].[Authors] ([AuthorId]) ON DELETE CASCADE
);
CREATE TABLE [dbo].[Categories] (
[CategoryId] INT IDENTITY (1, 1) NOT NULL,
[CategoryName] NVARCHAR (MAX) NULL,
CONSTRAINT [PK_Categories] PRIMARY KEY CLUSTERED ([CategoryId] ASC)
);
CREATE TABLE [dbo].[BookCategory] (
[BooksBookId] INT NOT NULL,
[CategoriesCategoryId] INT NOT NULL,
CONSTRAINT [PK_BookCategory] PRIMARY KEY CLUSTERED ([BooksBookId] ASC, [CategoriesCategoryId] ASC),
CONSTRAINT [FK_BookCategory_Books_BooksBookId] FOREIGN KEY ([BooksBookId]) REFERENCES [dbo].[Books] ([BookId]) ON DELETE CASCADE,
CONSTRAINT [FK_BookCategory_Categories_CategoriesCategoryId] FOREIGN KEY ([CategoriesCategoryId]) REFERENCES [dbo].[Categories] ([CategoryId]) ON DELETE CASCADE
);
GO
CREATE NONCLUSTERED INDEX [IX_BookCategory_CategoriesCategoryId]
ON [dbo].[BookCategory]([CategoriesCategoryId] ASC);
EF Core will automatically create a join table to represent the many-to-many relationship between the two entities. The name of the join table will be generated based on the names of the entities, in this case, it will be BookCategory.
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