Strange `undefined method model_url` error. Am I missing something with decent exposure? Is the namespace on the controller causing an issue?

preorders_controller.rb

class Api::PreordersController < ApplicationController

  respond_to :json
  expose :preorder, attributes: :preorder_params

  def create
    preorder.save
    respond_with preorder # does not work
    # respond_with preorder, location: api_preorder_url(preorder) # works
  end

  private

  def preorder_params
    params.permit(:email)
  end
end

preorders_controller_spec.rb

require 'spec_helper'

describe Api::PreordersController do

  describe '#create' do
    let(:params) { {email: 'm@m.com', format: :json} }
    it 'creates a new preorder' do
      expect{
        post :create, params
      }.to change{
        Preorder.count
      }.by(1)
    end
  end

end

routes.rb

  
  namespace :api do
    resources :preorders, only: [:create, :show]
  end

terminal.sh

$ spring rspec spec/controllers/api/preorders_controller_spec.rb 
  1) Api::PreordersController#create creates a new preorder
     Failure/Error: post :create, params: params, format: :json
     NoMethodError:
       undefined method `preorder_url' for #<Api::PreordersController:0x007facf46198f8>
     # ./app/controllers/api/preorders_controller.rb:8:in `create'
     # ./spec/controllers/api/preorders_controller_spec.rb:9:in `block (4 levels) in <top (required)>'
     # ./spec/controllers/api/preorders_controller_spec.rb:8:in `block (3 levels) in <top (required)>'
     # -e:1:in `<main>'

"post :create, params" should be "post :create, params, format: :json" since your action respond_to json