issue-179.rb

require 'mongoid'                                                          
                                                                           
Mongoid.configure do |config|                                              
  config.master = Mongo::Connection.new.db('testing')                      
end

class Parent                                                               
  include Mongoid::Document                                                
  field :name

  collection.remove
end

Parent.create                                                              
Parent.create(:name => "John")                                             

Parent.where(:name => nil).size # => 1

Parent.collection.find(:name => {'$in' => [nil]}).to_a.size # => 0
Parent.where(:name.in => [nil]).size # => 0
# mongo
# > use testing
# > db.parents.count({ name : { $in : [null] } }) # => 0