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

Question

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.