comment..rb

  def self.create_from_parent(parent_type, parent_id, comment_params, current_account)
    raise ActiveRecord::RecordNotFound unless POLYMORPHIC_ON.include?(parent_type)
    parent_type.constantize.find(parent_id).comments.create(comment_params.merge(account: current_account))
  end

controller.rb

  def create
    parent_resource = params[:commentable_type]
    parent_id = params[:commentable_id]

    @comment = Comment.create_from_parent(parent_resource, parent_id, comment_params, current_account)

    respond_with :dashboard, @comment
  end