Testing in Ember with Mirage - delete item without dependent items

In my Ember Application you can add, edit and delete events and todos and connect the todos with the events. The backend is a rails application. For testing I use Mirage. The models in Mirage have following relationships:

//Mirage event
import { belongsTo, hasMany, Model } from 'ember-cli-mirage';

export default Model.extend({
  user: belongsTo('user'),
  todos: hasMany('todo')
});

//Mirage todo
import { belongsTo, Model } from 'ember-cli-mirage';

export default Model.extend({
  user: belongsTo('user'),
  event: belongsTo('event')
});

In my acceptance test I create a user with an event and some todos connected with the event and the user. I visit the events page and click on the created event. You can select all todos connected with the event to delete them too. But in this case I just want to delete the event without the connected todos. Than I start the delete request by confirming.

//acceptance test events-test.js    
test('delete event without todo', function(assert) {
    const user = server.create('user'),
        event = server.create('event', {
            user
        }),
        todos = server.createList('todo', 4, { user, event });
    authenticateSession(this.application, {
        id: user.id,
        token: user.token
    });
    visit('/events');
    andThen(function() {
        assert.equal(find('td.fc-event-container').length, 5, 'There should be exactly one Event and four Todo');
        assert.equal(currentURL(), '/events', 'currentURL should be /events');
        click(find('td.fc-event-container a').eq(0));
        andThen(function() {
            click('button.delete');
            andThen(function() {
                click('button.confirm');
                andThen(function() {
                    assert.equal(find('td.fc-event-container').length, 4, 'There should be exactly zero Events but four Todo');
                    assert.equal(currentURL(), '/events', 'currentURL should be /events');
                });
            });
        });
    });
});

The delete route in the config.js:

 //Mirage config.js
     this.del('/events/:id');

In productive code everything works fine. But in the test the created event has zero todos as relations and after confirming it deletes the event and exactly one todo. error message