class Foo
  include Mongoid::Document
  embeds_many :bars
end

class Bar
  include Mongoid::Document
  embedded_in :foo

  field :name, :type => String
end

f = Foo.new
f.bars.build(:name => "Building")
f.save()
irb(main):018:0> f.bars.where(:name => /build/).count
=> 0

# I would expect this to match but it does not
irb(main):019:0> f.bars.where(:name => {:$not => /build/}).count
=> 0