mongoid/mongoid — pull request #3315 by Subskii (2013-10-10 08:49:40+00:00)
Title: Fix broken in/nin matchables where attribute is nil

```ruby
class ParentDoc
include Mongoid::Document

embeds_many :child_docs
end

class ChildDoc
include Mongoid::Document

field :attribute

embedded_in :parent_doc
end

ParentDoc.create!(child_docs: [ { attribute: nil } ] )

ParentDoc.where(:'child_docs.attribute'.in => [nil]) => 1
ParentDoc.where(:'child_docs.attribute'.in => [nil]).first.child_docs.where(:attribute.in => [nil]).count => 0
```