Question

2 entities: Member and Comment
Member has an ICollection<Comment> Comments

Whenever I use member.Comments.Take(x) EF produces a query that gets all the comments from database.
Is it supposed to be like that?
Is it because property is ICollection?
Is there a way to tell EF to factor in my Take(x) or should i refactor my code to use context.Comments.Where(c=>c.MemberId==member.Id).Take(x) and live with it?

Answer 1

The proxy classes generated by EF only provide lazy-loading for navigation properties, but they do not evaluate queries. Once you accessed the member.Comments property, the Comment-entities are loaded from the database and your query is applied in memory. To avoid this, you must get your comments in a query that is directly executed on the object-set (like the example you’ve already gave).

I believe this is by design, since you would have to return an IQueryable from the navigation property in order for the EF to intercept access to this property, but I suppose this isn’t covered aswell.

You’ve already described a way to handle this, although it isn’t pretty. Another option would be to somehow tell EF to partially load the property when you make the original query for the Member-object. I will look into that, but I can already think of one or two thinks that might go wrong with that approach.

Edit
After some research and trial and error I couldn’t come up with another approach, that could be executed directly on the DbSet<Member> rather than DbSet<Comment> and returns a Member object. I is possible using an anonymous object:

var query = from m in catalog.Members
            select new
            {
                Id = m.Id,
                Name = m.Name,
                Comments = m.Comments.Take(1)
            };

Which could then be translated into a Member-object in memory, but of course it wouldn’t be connected to the context in anyway (=no change tracking). In the sample query above I cannot create an instance of Member instead of an anonymous type, because EF can only create non-complex types (I’m guessing because the context knows that “Member” is an entity).