Include Entity of generic list property in ASP.NET EntityFramework Core

Question

I have a many-to-many relation between parents and childs using a relation table cause those relations are not automatically supported in EF Core yet:

class Parent{
    [Key]
    public int Id{get;set;}
    public List<ParentChild> ParentChilds{get;set;}
}

class Child{
    [Key]
    public int Id{get;set;}
    public List<ParentChild> ParentChilds{get;set;}
}

class ParentChild{
    public int ParentId{get;set;}
    public Parent Parent{get;set;}
    public int ChildId{get;set;}
    public Child Child{get;set;}
}

For editing the parent, I need to get ALL of his childs. Seems like a job for Include()

var db = new MyDbContext();
var parentWithChilds = db.Parents.Include(p => p.ParentChilds)
    .FirstOrDefault(p => p.Id == 1);

This gave me the list of ParentChild istances. But the Child entity of ParentChild isn't loaded automatically, so I only have the Id of the child, but not the Child object itself which is needed for me. I found ThenInclude which seems to be designed for such cases and from examples like this I did the following:

var parentWithChilds = db.Parents.Include(p => p.ParentChilds)
    .ThenInclude(p => p.Select(x => x.Child))
    .FirstOrDefault(p => p.Id == 1);

But it throws an exception:

The property expression 'p => {from ParentChild x in p select [x].Child => FirstOrDefault()}' is not valid. The expression should represent a property access: 't => t.MyProperty'.

So how can this be done? I would like to avoid unnecessary queries like fetching the entity manually this way:

user.ParentChilds.ForEach(pc => pc.Child = db.Childs.FirstOrDefault(x => x.Id == pc.ChildId));

Answer 1

Seems like I misunderstand the usage of ThenInclude since it refers to the sub-entity. Having a list its possible to define the entity to load also on lists like this:

var parentWithChilds = db.Parents.Include(p => p.ParentChilds)
    .ThenInclude(p => p.Child)
    .FirstOrDefault(p => p.Id == 1);

Visual Studio has issues showing those overload in intellisense, but its there and wouldn't result in errors.