gistfile1.rb

Models
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Invitation
has_many :comments

Comment
belongs_to :invitation


Controller
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class InvitationsController < ApplicationController
  def show
    @invitation = Invitation.find(params[:id])
    respond_with(@invitation)
  end
end


routes.rb
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resources :invitations do
    member do
      post 'create_comment'
    end
  end

View show.html.erb [http://localhost:3000/invitations/1]
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<p>
  <b>Title:</b>
  <%= @invitation.title %>
</p>

<%= simple_form_for @comment, :url => create_comment_invitation_path(@invitation) do |f| %>
  <%= f.input :comment, :label => 'Comment', :placeholder => 'Enter Comment' %>
  <%= link_to 'Submit', create_comment_invitation_path(@invitation) , :class => 'btn' %>
<% end %>