Laravel: Extending Models With Existing Relationships

Extending a model in Laravel can cause complications with existing relationships. I’ll use a User model for this example with a bunch of existing relationships to other models, like permissions or roles. And for reasons, we are unable to alter the User model and decide to create a new class to extend it. The new model is Author, which extends User.

Laravel’s base Model class will take our class name and primary key name, forming a foreign key using snake casing. For our user, this is obviously user_id, and should this not be the case we can override the foreign key name when we define our relationships in the model.

However, returning to the Author model from before, the inherited relationships from the user model are still expecting user_id. Since this is a different model though, the foreign key now becomes author_id. Breaking any inherited relationships from the User model that have not been manually overridden.

Luckily there’s a fix for this. Simply override the getForeignKey method in the Author model to return user_id.

public function getForeignKey() {
    return 'user_id';

Of course this assumes that the Author model hasn’t defined any new relationships using author_id as the foreign key.

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