class Subdoc
  include Mongoid::Document
  embedded_in :foo
  field :a
  field :b
end
class Foo
  include Mongoid::Document
  embeds_many :subdocs
end

f = Foo.new
f.subdocs.new(:a => 1, :b => 1)

f.subdocs.where(:a => 1).first
=> #<Subdoc _id: 57912b932774e24d68ce123f, a: 1, b: 1>

f.subdocs.where(:$elemMatch => {:a => 1}).first
=> nil