title: ThenInclude not working for an Entity Framework LINQ query
body: <p>I have a database model like this:</p>
  
  <pre><code>public class Customer
  {
      public int CustomerId{ get; set; }
  
      public int OrderId { get; set; }
      public ICollection&lt;Order&gt; Orders { get; set; }
  }
  
  public class Order
  {
      public int OrderId { get; set; }
      public int Amount { get; set; }
  
      public int ProductId { get; set; }
      public Product Product { get; set; }
  }
  
  public class Product
  {
      public int ProductId { get; set; }
      public string Name { get; set; }
  }
  </code></pre>
  
  <p>So we have a customer which can do an order. This order includes a product and this includes a name. I'm now trying to return the complete model with a linq statement like the following:</p>
  
  <pre><code>_db.Customer.Include(c =&gt; c.Orders).ThenInclude(o =&gt; o.Product).SingleOrDefaultAsync();
  </code></pre>
  
  <p>But <code>ThenInclude(o =&gt; o.Product)</code> doesn't function because Orders is an ICollection. Greetings from Germany and thanks in advance for any help.</p>