How to conditionally render template in Grape resources?

Suppose you need to conditionally render some template in a given endpoint of a Grape resource. There is a env data in Grape::API class that represents Rack environment of the request. env is just a simple Ruby hash and we can pass the template name to it's api.tilt.template key to render that template:

env['api.tilt.template'] = 'foo/bar.json'

Then let's define a render method in Base resourсe:

class Base < Grape::API

  def self.inherited(subclass)

    super



    subclass.instance_eval do

      helpers do

        def render(template_name)

          env['api.tilt.template'] = template_name

        end

      end

    end

  end

end

Now we can use it in the following way:

class ContactsResource < Base

  desc 'Create contact', http_codes: [[201, 'Created']]



  post '/contacts' do

    contact = Contact.new(params[:contact])



    if contact.save

      render 'v1/contacts/show.json'

    else

      error!({ errors: contact.errors.full_messages }, 422)

    end

  end

end