Rails API Server RSpec test always returns empty JSON

I have a problem with testing my Controllers. All tests are working fine, the Controllers are basic CRUD, but one #index action always returns an empty JSON body. I've tried everything and ran out of ideas, so maybe you can help me.

Here is my Code

Controller

class CarUsersController < ApplicationController

def index
  @car_users = CarUser.all

  render json: @car_users
end

Migration

class CreateCarUsers < ActiveRecord::Migration[5.0]
  def change
    create_table :car_users do |t|
      t.integer :car_id
      t.integer :owner_user_id
      t.integer :rental_user_id
      t.integer :service_user_id
      t.string :role

      t.timestamps
    end
  end
end

Model

class CarUser < ApplicationRecord

  rolify
  belongs_to :car
  belongs_to :owner_user
  belongs_to :rental_user
  belongs_to :service_user

  validates :role, presence: true
  validates :role, acceptance: { accept: ['owner', 'rental', 'service'] }

end

RSpec Factory

require 'faker'

FactoryGirl.define do
  factory :car_user do |p|

    p.owner_user_id { Faker::Number.digit }
    p.driver_user_id { Faker::Number.digit }
    p.service_user_id { Faker::Number.digit }
    p.car_id { Faker::Number.digit }
    p.role 'owner'

    end
end

And the RSpec-Test

...
it 'returns all car users' do
  FactoryGirl.create(:car_user, rental_user_id: 4, car_id: 2, role: 'rental')
    get :index
    puts response.body
    parsed_response = JSON.parse(response.body)

    expect(parsed_response[0]['id']).to eq(1)
    expect(parsed_response[0]['rental_user_id']).to eq(4)
    expect(parsed_response[0]['car_id']).to eq(2)
    expect(parsed_response[0]['role']).to eq('rental')
end
...

I always get the following error:

CarUsersController returns all car users
Failure/Error: expect(parsed_response[0]['rental_user_id'].to eq(4)

  expected: 4
       got: nil

  (compared using ==)