comments_controller.rb

class CommentsController < ApplicationController
  respond_to :html, :json
  
  def create
    @report = Report.find(params[:report_id])
    @comment = @report.comments.build(params[:comment])
    @comment.user = current_user
    respond_with @report, @comment, :methods => [:by_admin?]
  end
end

gistfile1.html

<%= form_for [@report, @report.comments.build], :remote => true, :html => {:'data-type' => 'json'} do |f| %>
  <%= f.text_area :content, :rows => '3', :placeholder => 'Type your comment here.' %>
  <%= f.submit %>
<% end %>

gistfile2.coffee

$(document).on 'ajax:success', '#new_comment', (e, data, status, xhr) ->
    console.log data
    #Object
    #  by_admin?: true
    #  content: ""
    #  created_at: null
    #  id: null
    #  report_id: 1
    #  updated_at: null
    #  user_id: 1
    #  __proto__: Object