Conditional Include() in Entity Framework

Question

I have seen a few answers to similar questions, however I cannot seem to work out how to apply the answer to my issue.

var allposts = _context.Posts
            .Include(p => p.Comments)
            .Include(aa => aa.Attachments)
            .Include(a => a.PostAuthor)
            .Where(t => t.PostAuthor.Id == postAuthorId).ToList();

Attachments can be uploaded by the Author (type Author) or Contributor (type Contributor). What I want to do, is only get the Attachments where the owner of the attachment is of type Author.

I know this doesn't work and gives an error:

.Include(s=>aa.Attachments.Where(o=>o.Owner is Author))

I've read about Filtered Projection here

EDIT - link to article: : http://blogs.msdn.com/b/alexj/archive/2009/10/13/tip-37-how-to-do-a-conditional-include.aspx,

but I just can't get my head around it.

I don't want to include the filter in the final where clause as I want ALL posts, but I only want to retrieve the attachments for those posts that belong to the Author.

EDIT 2: - Post schema requested

public abstract class Post : IPostable
{

    [Key]
    public int Id { get; set; }

    [Required]
    public DateTime PublishDate { get; set; }

    [Required]
    public String Title { get; set; }

    [Required]
    public String Description { get; set; }

    public Person PostAuthor { get; set; }
    public virtual ICollection<Attachment> Attachments { get; set; }
    public List<Comment> Comments { get; set; }
}

Answer 1

From the link you posted I can confirm that trick works but for one-many (or many-one) relationship only. In this case your Post-Attachment should be one-many relationship, so it's totally applicable. Here is the query you should have:

//this should be disabled temporarily
_context.Configuration.LazyLoadingEnabled = false;
var allposts = _context.Posts.Where(t => t.PostAuthor.Id == postAuthorId)
                       .Select(e => new {
                           e,//for later projection
                           e.Comments,//cache Comments
                           //cache filtered Attachments
                           Attachments = e.Attachments.Where(a => a.Owner is Author),
                           e.PostAuthor//cache PostAuthor
                        })
                       .AsEnumerable()
                       .Select(e => e.e).ToList();