comment..rb

  def self.create_from_parent(parent_type, parent_id, comment_params, current_account)
    if POLYMORPHIC_ON.include?(parent_type)
      valid = parent_type.constantize.exists?(parent_id)
    else
      valid = false
    end

    if valid
      parent_type.constantize.find(parent_id).comments.create(comment_params.merge(account: current_account))
    else
      false
    end
  end

controller.rb

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

    @comment = Comment.create_from_parent(params[:commentable_type], params[:commentable_id], comment_params, current_account)

    if @comment
      respond_with :dashboard, @comment
    else
      # Explode?
    end
  end