class Foo
  include Mongoid::Document
  embeds_one :bar  
  field :a, type: Integer
end

class Bar
  include Mongoid::Document
  embedded_in :foo
  field :b, type: Integer
end


f=Foo.new(:a=>10)
b=Bar.new(:b=>20)
f.bar=b
f.save


Foo.where(:"a.gt"=>5).count
 # => 0 WHY?
Foo.where(:a.gt=>5).count
 # => 1 

 Foo.where("a"=>10).count
 => 1 
 Foo.where(:a=>10).count
 => 1 


Foo.where(:bar.b.gt=>5).count
 # NoMethodError: undefined method `b' for :bar:Symbol
Foo.where(:"bar.b.gt"=>5).count
 # => 0