conversations_controller.rb

...
def create
  recipient_id = Post.find(params[:post_id]).user.id
  conversation = Private::Conversation.new(sender_id: current_user.id, 
                                           recipient_id: recipient_id)
  if conversation.save
    Private::Message.create(user_id: recipient_id, 
                            conversation_id: conversation.id, 
                            body: params[:message_body])
    respond_to do |format|
      format.js {render partial: 'posts/show/contact_user/message_form/success'}
    end
  else
    respond_to do |format|
      format.js {render partial: 'posts/show/contact_user/message_form/fail'}
    end
  end
end
...