How can you use a LINQ Lambda Where Property Count = Something

I have an Entity object, Item, that looks like this..

public class Item()
{
    IEnumerable<Category> Categories { get; set; }
}

and I am trying to get a list of items that have a 0 count

var unassigned = db.Items.Where(i => i.Categories.Count() == 0);

or

var unassigned = db.Items.Where(i => i.Categories.Any());

but both throw the error... "The specified type member 'Categories' is not supported in LINQ to Entities. Only 开发者_StackOverflowinitializers, entity members, and entity navigation properties are supported."

What is this error telling me and how can I query for what I am looking for?

Is Category an entity as well that is tracked by EF? EF doesn't seem to think it is. Are you using POCOs? Typically navigation properties that are 1-to-many need to be represented with ICollection<T> not IEnumerable.

Basically EF is saying it doesn't know how to turn db.Items.Where(i => i.Categories.Count() == 0) into SQL, because it's unsure what Categories is in relation to the database.

It's also possible you will need to Include("Categories") on the EF call, but I think you have more fundamental problems than this.

Try

var unassigned = db.Items.Categories.Where(c=>c.Count() ==0);