In Rails, how do you render JSON using a view?

Question

You should be able to do something like this in your respond_to block:

respond_to do |format|
    format.json 
    render :partial => "users/show.json"
end

which will render the template in app/views/users/_show.json.erb.

Try adding a view users/show.json.erb This should be rendered when you make a request for the JSON format, and you get the added benefit of it being rendered by erb too, so your file could look something like this

{
    "first_name": "<%= @user.first_name.to_json %>",
    "last_name": "<%= @user.last_name.to_json %>"
}

As others have mentioned you need a users/show.json view, but there are options to consider for the templating language...

ERB

Works out of the box. Great for HTML, but you'll quickly find it's awful for JSON.

RABL

Good solution. Have to add a dependency and learn its DSL.

JSON Builder

Same deal as RABL: Good solution. Have to add a dependency and learn its DSL.

Plain Ruby

Ruby is awesome at generating JSON and there's nothing new to learn as you can call to_json on a Hash or an AR object. Simply register the .rb extension for templates (in an initializer):

ActionView::Template.register_template_handler(:rb, :source.to_proc)

Then create the view users/show.json.rb:

@user.to_json

For more info on this approach see http://railscasts.com/episodes/379-template-handlers

RABL is probably the nicest solution to this that I've seen if you're looking for a cleaner alternative to ERb syntax. json_builder and argonaut, which are other solutions, both seem somewhat outdated and won't work with Rails 3.1 without some patching.

RABL is available via a gem or check out the GitHub repository; good examples too

https://github.com/nesquena/rabl

Just to update this answer for the sake of others who happen to end up on this page.

In Rails 3, you just need to create a file at views/users/show.json.erb. The @user object will be available to the view (just like it would be for html.) You don't even need to_json anymore.

To summarize, it's just

# users contoller
def show
  @user = User.find( params[:id] )
  respond_to do |format|
    format.html
    format.json
  end
end

and

/* views/users/show.json.erb */
{
    "name" : "<%= @user.name %>"
}

Just add show.json.erb file with the contents

<%= @user.to_json %>

Sometimes it is useful when you need some extra helper methods that are not available in controller, i.e. image_path(@user.avatar) or something to generate additional properties in JSON:

<%= @user.attributes.merge(:avatar => image_path(@user.avatar)).to_json %>

This is potentially a better option and faster than ERB: https://github.com/dewski/json_builder