How do I render a partial of a different format in Rails?

I'm trying to generate a JSON response that includes some HTML. Thus, I have /app/views/foo/bar.json.erb:

{
  someKey: 'some value',
  someHTML: "<%= h render(:partial => '/foo/baz') -%>"
}

I want it to render /app/views/foo/_baz.html.erb, but it will only render /app/views/foo/_baz.json.erb. Passing :format => 'html' doesn't help.

Beginning with Rails 3.2.3, when calling render :partial (only works outside of the respond_to block).

render formats: [ :html ]

instead of

render format: 'html'

What's wrong with

render :partial => '/foo/baz.html.erb'

? I just tried this to render an HTML ERB partial from inside an Atom builder template and it worked fine. No messing around with global variables required (yeah, I know they have "@" in front of them, but that's what they are).

Your with_format &block approach is cool though, and has the advantage that you only specify the format, whereas the simple approach specifies the template engine (ERB/builder/etc) as well.

Rails 4 will allow you to pass a formats parameter. So you can do

render(:partial => 'form', :formats => [:html])} 

Note you can do something similar in Rails 3 but it wouldn't pass that format to any sub partials (if form calls other partials).

You can have the Rails 4 ability in Rails 3 by creating config/initializers/renderer.rb:

class ActionView::PartialRenderer
  private
  def setup_with_formats(context, options, block)
    formats = Array(options[:formats])
    @lookup_context.formats = formats | @lookup_context.formats
    setup_without_formats(context, options, block)
  end

  alias_method_chain :setup, :formats
end

See http://railsguides.net/2012/08/29/rails3-does-not-render-partial-for-specific-format/

For Rails 3, the with_format block works, but it's a little different:

  def with_format(format, &block)
    old_formats = formats
    self.formats = [format]
    block.call
    self.formats = old_formats
    nil
  end

Building on roninek's response, I've found the best solution to be the following:

in /app/helpers/application.rb:

def with_format(format, &block)
  old_format = @template_format
  @template_format = format
  result = block.call
  @template_format = old_format
  return result
end

In /app/views/foo/bar.json:

<% with_format('html') do %>
  <%= h render(:partial => '/foo/baz') %>
<% end %>

An alternate solution would be to redefine render to accept a :format parameter.

I couldn't get render :file to work with locals and without some path wonkiness.

In Rails 3, the View has a formats array, which means you can set it to look for [:mobile, :html]. Setting that will default to looking for :mobile templates, but fall back to :html templates. The effects of setting this will cascade down into inner partials.

The best, but still flawed way, that I could find to set this was to put this line at the top of each full mobile template (but not partials).

<% self.formats = [:mobile, :html] %>

The flaw is that you have to add that line to multiple templates. If anyone knows a way to set this once, from application_controller.rb, I'd love to know it. Unfortunately, it doesn't work to add that line to your mobile layout, because the templates are rendered before the layout.

Just elaborating on what zgchurch wrote:

• taking exceptions into account
• returning the result of the called block

Thought it might be useful.

def with_format(format, &block)
  old_formats = formats
  begin
    self.formats = [format]
    return block.call
  ensure
    self.formats = old_formats
  end
end

You have two options:

1. use render :file

   render :file => "foo/_baz.json.erb"

2. change template format to html by setting @template_format variable

   <% @template_format = "html" %> <%= h render(:partial => '/foo/baz') %>

I had a file named 'api/item.rabl' and I wanted to render it from an HTML view so I had to use:

render file: 'api/item', formats: [:json]

(file because the file have no underscore in the name, formats and not format (and passes and array))

It seems that passing a formats option will render it properly in newer Rails version, at least 3.2:

{
  someKey: 'some value',
  someHTML: "<%= h render('baz', formats: :html) -%>"
}

I came across this thread when I was trying to render an XML partial in another xml.builder view file. Following is a nice way to do it

xml.items :type => "array" do
    @items.each do |item|
        xml << render(:partial => 'shared/partial.xml.builder', :locals => { :item => item })
    end
end

And yeah... Full file name works here as well...