How to intentionally fail $model->save() for testing? - yii2

Question

I have this code in my controller class:
public function actionRevoke($id) {
$model = Token::findById($id);
$model->revoked_at = new Expression('NOW()');
if ($model->save()) {
Yii::$app->getSession()->setFlash('success', 'Revoked token');
} else {
Yii::$app->getSession()->setFlash('danger', 'Failed to revoke token');
}
return $this->render('revoke',['model' => $this]);
}
Now, running CodeCeption with code coverage, it tells me, the part with setFlash('danger',...) is not covered by tests.
How can I intentionally fail the $model->save() to trigger the setFlash('danger,...) instruction?
I would really like it, if I do not need to add extra code to my Controller exclusively for testing. So in particular, I don't want something like if ($model->save() && !defined('TEST_FAIL_SAVE')) { if it can be avoided.

Answer 1

The solution was (as suggested by #scaisEdge) to use validation rules. I added a rule, which enforces the length of $model->token to exactly 36. My application generates only tokens exactly 36 chars long. In my test, I use update the row in the database to show abcd as token, which is clearly less than 36 chars.
As this change is made on database level, it bypasses the validation of Yii and is committed to the database.
During the test, it changes the value of $model->revoked_at and attempts to save it, but fails the validation because $model->token is too short.
Here is the relevant part in the test function, the code of the controller was not changed:
public function revokeToken(FunctionalTester $I){
// login and navigation to relevant page
Yii::$app->db->createCommand()->update('tokens',['token' => 'abcd'],['id' => 201 ])->execute();
$I->click('a.btn.btn-danger'); /* click the 'revoke' button, this triggers actionRevoke($id) */
$I->seeFlash('Failed to revoke token'); /* expect a flash message */
}