[Bug] `LinkTo` with incomplete model failing in rendering tests

[Turbo87]

🐞 Describe the Bug

We have a few components that use LinkTo components but are only used on subroutes with dynamic segments and have not explicitly added @model (e.g. only used within /organizations/:org_id). This works fine in production and up until Ember 3.24 this also worked fine in application and rendering tests.

With 3.24 Ember now automatically starts the routing layer whenever something on the router service is accessed, and now the LinkTo component is complaining that we did not pass the necessary models when any previous thing in the template for example accesses router.currentRouteName.

🔬 Minimal Reproduction

Describe steps to reproduce. If possible, please, share a link with a minimal reproduction.

// router.js

Router.map(function() {
  this.route('parent', { path: 'parent/:parent_id' }, function () {
    this.route('child');
  });
});

test('broken', async function(assert) {
  // access a property on the router
  this.owner.lookup('service:router').currentRouteName;

  // render a LinkTo with incomplete model
  await render(hbs`<LinkTo @route="parent.child">Link</LinkTo>`);
});

😕 Actual Behavior

Assertion Failed: You attempted to generate a link for the "parent.child" route, but did not pass the models required for generating its dynamic segments. Cannot read property 'shouldSupercede' of undefined

🤔 Expected Behavior

The LinkTo should render without an href like it did prior to Ember 3.24, or the router.currentRouteName access should not have the side-effect of automatically starting the routing layer, which is causing these issues.

🌍 Environment

• Ember: 3.24

➕ Additional Context

somewhat related:

• Lazily setup the router in non-application tests #19080
• fix(link-manager): Use private router API to query currentURL in a side-effect-free way buschtoens/ember-link#518
• Fix Ember 3.24 compatibility buschtoens/ember-link#507

/cc @rwjblue @xg-wang

[xg-wang]

Sorry for the delay, I won't have much time until the weekend to look into the fix

[rwjblue]

Chatted about this with @xg-wang earlier, I think what we should do is to change this guard:

ember.js/packages/@ember/-internals/routing/lib/services/routing.ts

Lines 57 to 60 in e25c0f2

	// return early when the router microlib is not present, which is the case for {{link-to}} in integration tests
	if (!router._routerMicrolib) {
	return;
	}

To be something like:

    let router = this.router;
    // return early when the router microlib is not present, which is the case for {{link-to}} in integration tests
    if (!router._hasBeganInitialRouting) {
      return;
    }

Then update:

ember.js/packages/@ember/-internals/routing/lib/system/router.ts

Lines 407 to 418 in e25c0f2

	startRouting() {
	if (this.setupRouter()) {
	let initialURL = get(this, 'initialURL');
	if (initialURL === undefined) {
	initialURL = get(this, 'location').getURL();
	}
	let initialTransition = this.handleURL(initialURL);
	if (initialTransition && initialTransition.error) {
	throw initialTransition.error;
	}
	}
	}

To set that flag.

[rwjblue]

Fixed by #19387, released in 3.25.1 and 3.24.2.