public class A
{
    public string Name { get; set }
    public ICollection<B> b { get; set; }
}

public class B
{
    public string Name { get; set }
    public ICollection<A> a { get; set; }
}

Since .NET 5.0 we don't need to create a join class, my data has been successfully inserted into tables A and B as well as the join table (which has PK for A and B).

But when I try to retrieve data like:

DBConext.A.b.count

I get 0. I double-checked my database and found data is there, both tables A, B, and the join table AB, I know this probably a small issue, someone please help me

CodePudding user response：

You need to use Include method to specify related data to be included in query results. But you cannot simply using DBContext.A.Include(a => a.b).b.count to get the count. Due to your two tables contain many-to-many relationship.

If you want to get specific A class's(by specific id or name...) related B class count, you can use:

public class TestController : ControllerBase
{
    private readonly Dotnetcore5_0Context _context;

    public TestController(Dotnetcore5_0Context context)
    {
        _context = context;
    }
    [HttpGet]
    public void Get()
    {
        //get the count of B class if A's Name equals to aa
        var data = _context.A.Include(a => a.b)
                             .Where(a=>a.Name=="aa").ToList()
                             .Select(a => a.b.Count()).ToList();

    }
}

If you want to get all the A class's related B class count, you can use:

public class TestController : ControllerBase
{
    private readonly Dotnetcore5_0Context _context;

    public TestController(Dotnetcore5_0Context context)
    {
        _context = context;
    }
    [HttpGet]
    public void Get()
    {
        var data = _context.A.Include(a => a.b).ToList()
                             .Select(a => a.b.Count()).ToList();

    }
}

CodePudding user response：

Additionally to Rena answer, you can simplify a lot of things if you use lazy proxies.

In DbContext, add following:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder.UseLazyLoadingProxies();
    ...
} 

This is may be useful when you're fetching relations for one/two entities. If you want to load relations for many objects, it's better to use Include, because data will load from one DB request, not hundreds.