Rails: Why can't you set an association to nil in a where clause?

I have photos that belong to collections and users. Photos always belong to a user, but may not be assigned to a collection.

In my controller, this works perfectly:

@collection_photos = Photo.where( :collection => @collection, :user => current_user )

However, this fails...

@other_photos = Photo.where( :collection => nil, :user => current_user )

...but this works:

@other_photos = Photo.where( :collection_id => nil, :user => current_user )

When collection is set to nil I get this error message: No attribute named 'collection' exists for table photos.

If I pass an object, it knows to search for collection_id from the symbol :collection, but if I don't pass an object it doesn't seem to be aware of the association.

Am I understanding this correctly? Could anyone explain a little better why :collection=>nil doesn't work?