Rails: return only json - ruby-on-rails

Question

In my app there is a list of items which you can upvote. I want to make these votes with AJAX calls.
This is the view:
<ul class="list-groups">
<% #questions.each do |question| %>
<li class="list-group-item">
<%= link_to question.description, question_path(question) %>
<%= form_for(question, :url => url_for(:controller => 'vote', :action => 'vote'), method: :post, html: { class: 'form-inline' }) do |f| %>
<%= f.submit 'Up vote', class: "btn btn-default" %>
<%= f.hidden_field :id, :value => question.id %>
<% end %>
</li>
<% end %>
</ul>
And this the method that does it:
class VoteController < ApplicationController
respond_to :json
def vote
question_id = params[:question][:id]
user_id = current_user.id
vote = Vote.where(["question_id = :q", { q: question_id }]).where(["user_id = :u", { u: user_id }])
respond_to do |format|
if vote.nil?
#vote = Vote.new
#vote.question_id = question_id
#vote.user_id = user_id
#vote.save
format.html { render '/home/index' }
format.json { render :json => { :status => 'ok' } }
else
format.html { render '/home/index' }
format.json { render :json => { :status => 'failed', :msg => 'You already voted' } }
end
end
end
end
If I don't include this format.html { render '/home/index' } I am getting this error:
ActionController::UnknownFormat in VoteController#vote
But I don't want to render the page again, I am just loading the pieces of html that will change after the action with jQuery and ajax.
How can I respond only with the json?

Answer 1

Use respond_with instead of respond_to in your controller action.
respond_with do |format|
respond_to at the top of your controller is designed to work with respond_with in your controller action, whereas respond_to in your controller action ignores the respond_to that you've defined at the top of your controller.
Also make sure you are making a remote call, instead of a normall one if you want your request to go through AJAX.
<%= form_for(question, :url => url_for(:controller => 'vote', :action => 'vote'), method: :post, remote: true, html: { class: 'form-inline' }) do |f| %>
Note the remote: true part that is added as an argument to the form_for helper.

Answer 2

You need to include remote: true in form_for in order to make the call requests AJAX instead of HTML.
<%= form_for(question, :url => url_for(:controller => 'vote', :action => 'vote'), remote: true, method: :post, html: { class: 'form-inline' }) do |f| %>
Check out the Working with JavaScript in Rails documentation for more information.