ctrl.rb

  def create
    parent_resource = params[:comment_commentable_type].constantize
    parent_id = params[:comment_commentable_id]
    
    # both of the above values are empty because both params are not passed as parameters.

    @comment = parent_resource.find(parent_id).comments.create(comment_params.merge(account: current_account))
     respond_with :dashboard, @comment
  end
  
  private
  
    def comment_params
      params.require(:comment).permit(:comment_markdown)
    end

fields.html.erb

  <%= f.hidden_field :commentable_type, value: parent.class %>
  <%= f.hidden_field :commentable_id, value: parent.id %>