EF Core 5 检索数据库多对多记录'值不能是 null。 （参数'源'）'

Question

As you know in Entity Framework Core 5 has the possibility to create a Many-to-Many relationship without explicitly mapping the join table. I have followed This Example and build successfully as described with this following model:

public class Movie
{
    public int MovieId { get; set; }
    public string Name{ 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; }
}

This is 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; }

}

then inserted some movie 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();

}

I'm adding some code after context.SaveChanges();:

var ReadMovie = context.Movies.ToList();
    foreach (var movie in ReadMovie)
    {
        var genres = String.Join(",",movie.Genres.Select(s => s.GenreName).ToArray());
        Debug.Print(String.Format("Move: {0}, Genres: {1}", movie.Name, genres));
    }

This is the error I'm facing that says movie.Genres is null :

System.ArgumentNullException: 'Value cannot be null. (Parameter 'source')'

I've already added new List<Genre>() while insertion, didn't I?

Answer 1

ReadMovie is containing only MovieId and Name, the List is null... the way that I know is creating a View which is joining Movies and Genres and there you can fill List the other way is:

var ReadMovie = context.Movies.Include(t=>t.Genres).FirstOrDefaultAsync(t => t.MovieId== id).ToList();