Conditional Include() in Entity Framework [duplicate]

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

EF Core 5.0 is introducing Filtered Include soon.

var blogs = context.Blogs     .Include(e => e.Posts.Where(p => p.Title.Contains("Cheese")))     .ToList(); 

Reference: https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-5.0/whatsnew#filtered-include

Answer 2

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();