Rails 5.1 API - incorrect url_for with a nested resource

Given this route structure:

routes:

  namespace :api do
    resources :templates do
      resources :template_items    
    end
  end

I scaffolded the template_items controller (but cannot recall exactly what I specified) but it gave me this create action:

  # POST /template_items
  def create
    @template_item = TemplateItem.new(template_item_params)

    if @template_item.save
      render json: @template_item, status: :created, location: @template_item
    else
      render json: @template_item.errors, status: :unprocessable_entity
    end
  end

The location throws an error:

NoMethodError (undefined method `template_item_url' 
for #<Api::TemplateItemsController:0x007fe84774fba8>

So I changed it to:

url_for([:api, @template_item])

which failed also:

NoMethodError (undefined method `api_template_item_url'

So then I tried:

url_for([:api, :template, @template_item])

which failed with this:

ActionController::UrlGenerationError 
(No route matches 
{:action=>"show", 
:controller=>"api/template_items", 
:template_id=>#<TemplateItemid: "b4a3921b-dcae-4902-aaf7-b2ef40e13707", 
template_id: "b2f22587-a36e-469f-9485-45b7d60c0120", 
content: nil, 
is_completed: false, 
item_type: "item", 
sort: 7, 
created_at: "2017-10-22 11:37:17", 
updated_at: "2017-10-22 11:37:17">}, 
missing required keys: [:id]):

I've read the docs, but it does not appear to explain how to do this for nested resources.

What is the correct way to generate the location url for a nested resource?

Not at all obvious from the docs, but I figured it out:

location: url_for([:api, @template_item.template, @template_item])

is how to do it.