Ran into an issue when iterating over a many association in a
before_save callback on a document. For an association named foos,
`foos.inspect` would show the items but `foos.each` would never yield to
the given block since it appears to be empty.
The original issue I was tracking down was that instead of not yielding
it was yielding instances of Mongo::Cursor instead of the association
object. I haven't been able to recreate that case.