EF Core FindAsync returns only root object

Question

I have a many to many relation and I am using ASP.Net Core2.2 MVC with EF.

From what I read I understand that at the current time I have to create a CLR class for join table.

My code:

Models:

public class Post
{
    public Post()
    {
        Categories= new List<PostCategory>();
    }
    public int Id { get; set; }
    [Required]
    public string Title { get; set; }
    [Required]
    public string Description { get; set; }
    public List<PostCategory> Categories { get; set; }
}

public class Category
  {
      public Category()
      {
          Posts= new List<PostCategory>();
      }
      public int Id { get; set; }
      [Required]
      public string Name { get; set; }
      public ICollection<PostCategory> Posts { get; set; }
    }
public class PostCategory
{
    public int PostId { get; set; }
    public Post Post { get; set; }

    public int CategoryId { get; set; }
    public Category Category { get; set; }
}

DBContext:

 public class MyDBContext:DbContext
{
    public MyDBContext()
    {

    }
    public MyDBContext(DbContextOptions<MyDBContext> options) : base(options)
    {

    }
    public DbSet<Post> Post { get; set; }

    public DbSet<Category> Category { get; set; }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<PostCategory>()
            .HasKey(x => new { x.PostId, x.CategoryId });
        modelBuilder.Entity<PostCategory>()
            .HasOne(x => x.Post)
            .WithMany(m => m.Categories)
            .HasForeignKey(x => x.PostId);
        modelBuilder.Entity<PostCategory>()
            .HasOne(x => x.Category)
            .WithMany(e => e.Posts)
            .HasForeignKey(x => x.CategoryId);
    }
}

Create Post Code:

var categories= new List<PostCategory>();
foreach (var category in model.SelectedCategories)
 {
  categories.Add(new PostCategory()
   {
      CategoryId= category
    });
 }
model.Post.Categories.AddRange(categories);
_context.Post.Add(model.Post);
await _context.SaveChangesAsync();

When I create a post I can see in database that I have Post data, and in PostCategory table I can see PostId CategoryId as they should be.

My issue is when I try to get a post data using:

var post = await _context.Post.FindAsync(id);

post.Categories count is always 0, what am I missing?

Answer 1

You need to write your query as follows to eager load the related Categories with Post:

var post =  await _context.Post.Include(p => p.Categories).FirstOrDefaultAsync(p => p.Id == id);

Here is the more details about Loading Related Data in EF core