def to_subquery
      collection = model.all(merge(:fields => model_key))
      Conditions::Operation.new(:and, Conditions::Comparison.new(:in, self_relationship, collection))
    end
def to_subquery
      collection = model.all(merge(:fields => model_key))
      Conditions::Operation.new(:and, Conditions::Comparison.new(:in, self_relationship, collection))
    end
Transform Query into subquery conditions

 @return [AndOperation]
   a subquery for the Query

 @api private