title: EF Core ICollection does not contain a definition for 'Include'
body: <p>I have a head and a details model with navigation properties between them. I'd like to get head records with details included. This works fine but the detail records I got doesn't contain all properties I need so I tried to add Include().</p>
  
  <p>EF Core says <code>ICollection&lt;RequestDetail&gt; does not contain a definition for 'Include'</code>. I have already tried with List&lt;> navigation types and <code>Microsoft.EntityFrameworkCore</code> and <code>System.Linq</code> are both in my usings.</p>
  
  <p>My models:</p>
  
  <pre><code>public class RequestHead
  {
      public string Id { get; set; }
      public DateTime CreateDate { get; set; }
      public int CreateUserId { get; set; }
      [ForeignKey("CreateUserId")]
      public User CreateUser { get; set; }
  
      public virtual ICollection&lt;RequestDetail&gt; Details { get; set; }
  }
  public class RequestDetail
  {
      [Key]
      public int Id { get; set; }
  
      public string RequestHeadId { get; set; }        
      [ForeignKey("RequestHeadId")]
      public RequestHead RequestHead { get; set; }
  
      public DateTime CreateDate { get; set; }
      public int CreateUserId { get; set; }
      [ForeignKey("CreateUserId")]
      public User CreateUser { get; set; }
  }
  </code></pre>
  
  <p>Select:</p>
  
  <pre><code> var requests = (from r in _ctx.RequestHeads
                  select new RequestDTO {
                      AcceptDate = r.AcceptDate,
                      AcceptUser = r.AcceptUser,
                      AcceptUserId = r.AcceptUserId,
                      Details = r.Details != null ? 
   r.Details.Include(x =&gt; x.BuyerUser).Select(x =&gt; new 
   RequestDetailDTO(x, x.Attachments.ToArray(), x.Product)).ToArray() : null});
  </code></pre>
  
  <p><strong>Update#1:</strong></p>
  
  <p>This is not a Json.ReferenceLoopHandling problem. It is already ignored in startup.cs and my problem is that I can't use Include() even in my code not just not having the neccessary properties in the json.</p>