rails 3 - How to render a PARTIAL as a Json response - ruby-on-rails

Question

I want to do something like this:
class AttachmentsController < ApplicationController
def upload
render :json => { :attachmentPartial => render :partial => 'messages/attachment', :locals => { :message=> #message} }
end
Is there a way to do this? render a Partial inside a JSON object? thanks

Answer 1

This should work:
def upload
render :json => { :attachmentPartial => render_to_string('messages/_attachment', :layout => false, :locals => { :message => #message }) }
end
Notice the render_to_string and the underscore _ in before the name of the partial (because render_to_string doesn't expect a partial, hence the :layout => false too).
UPDATE
If you want to render html inside a json request for example, I suggest you add something like this in application_helper.rb:
# execute a block with a different format (ex: an html partial while in an ajax request)
def with_format(format, &block)
old_formats = formats
self.formats = [format]
block.call
self.formats = old_formats
nil
end
Then you can just do this in your method:
def upload
with_format :html do
#html_content = render_to_string partial: 'messages/_attachment', :locals => { :message => #message }
end
render :json => { :attachmentPartial => #html_content }
end

Answer 2

This question is a bit old, but I thought this might help some folks.
To render an html partial in a json response, you don't actually need the with_format helper as explained in mbillard's answer. You simply need to specify the format in the call to render_to_string, like formats: :html.
def upload
render json: {
attachmentPartial:
render_to_string(
partial: 'messages/attachment',
formats: :html,
layout: false,
locals: { message: #message }
)
}
end

Answer 3

In Rails 6 I think this might be a little different from the accepted answer. I don't think you need to set the underscore in the partial name. This worked for me:
format.json {
html_content = render_to_string(partial: 'admin/pages/content', locals: { page: #page }, layout: false, formats: [:html])
render json: { attachmentPartial: html_content }
}