Earlier in Entity Framework, the many-to-many relationship was classified as two one-to-many relationships. To make it work the developer must create a joining entity class.
Now in Entity Framework Core 5.0, it will have full support for many-to-many relations without explicitly mapping the join table.
The navigation properties skip the join table and directly point to the other entity.
It will result in writing cleaner queries and simplify the use of the query result.
Let's consider the following model.
public class Movie
{
public int MovieId { get; set; }
public string Name{ get; set; }
public Actor Actor { get; set; }
public List<Genre> Genres { get; set; }
}
public class Genre
{
public int GenreId { get; set; }
public string GenreName { get; set; }
public List<Movie> Movies{ get; set; }
}
As you can see that Movie class contains a collection of Genres, and Genre class contains a collection of Movies. EF Core 5.0 recognizes this as a many-to-many relationship by convention and there is no need for configuration in OnModelCreating.
public class MyEntityContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder opBuilder)
{
opBuilder.UseSqlServer("Data Source=(localdb)\\ProjectsV13;Initial Catalog=MyContextDB;");
}
public DbSet<Movie> Movies { get; set; }
public DbSet<Genre> Genres { get; set; }
}
When you create migration or call the EnsureCreatedmethod, it will create the following tables including the join table.
CREATE TABLE [dbo].[Movies] (
[MovieId] INT IDENTITY (1, 1) NOT NULL,
[Name] NVARCHAR (MAX) NULL,
CONSTRAINT [PK_Movies] PRIMARY KEY CLUSTERED ([MovieId] ASC)
);
CREATE TABLE [dbo].[Genres] (
[GenreId] INT IDENTITY (1, 1) NOT NULL,
[GenreName] NVARCHAR (MAX) NULL,
CONSTRAINT [PK_Genres] PRIMARY KEY CLUSTERED ([GenreId] ASC)
);
CREATE TABLE [dbo].[GenreMovie] (
[GenresGenreId] INT NOT NULL,
[MoviesMovieId] INT NOT NULL,
CONSTRAINT [PK_GenreMovie] PRIMARY KEY CLUSTERED ([GenresGenreId] ASC, [MoviesMovieId] ASC),
CONSTRAINT [FK_GenreMovie_Genres_GenresGenreId] FOREIGN KEY ([GenresGenreId]) REFERENCES [dbo].[Genres] ([GenreId]) ON DELETE CASCADE,
CONSTRAINT [FK_GenreMovie_Movies_MoviesMovieId] FOREIGN KEY ([MoviesMovieId]) REFERENCES [dbo].[Movies] ([MovieId]) ON DELETE CASCADE
);
Let's insert some movies and genres.
using (var context = new MyEntityContext())
{
context.Database.EnsureCreated();
var comedy = new Genre() { GenreName = "Comedy" };
var action = new Genre() { GenreName = "Action" };
var horror = new Genre() { GenreName = "Horror" };
var scifi = new Genre() { GenreName = "Sci-fi" };
context.AddRange(
new Movie() { Name = "Avengers", Genres = new List<Genre>() { action, scifi } },
new Movie() { Name = "Satanic Panic", Genres = new List<Genre>() { comedy, horror } });
context.SaveChanges();
}
EF will then automatically create rows in the join table.
