class Address 
  include Mongoid::Document 
  field :name 
  field :tags, :type => Array 
end

Address.where :tags => nil
NoMethodError: undefined method `new' for NilClass:Class
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/serialization.rb:53:in `mongoize'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/selector.rb:116:in `typecast_value_for'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/selector.rb:88:in `try_to_typecast'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/selector.rb:33:in `[]='
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/inclusion.rb:170:in `block (2 levels) in where'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/inclusion.rb:161:in `each_pair'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/inclusion.rb:161:in `block in where'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/inclusion.rb:154:in `tap'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/criterion/inclusion.rb:154:in `where'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.1.7/lib/mongoid/finders.rb:14:in `where'
        from (irb):16
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands/console.rb:44:in `start'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands/console.rb:8:in `start'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands.rb:23:in `<top (required)>'
        from script/rails:6:in `require'
        from script/rails:6:in `<main>'