Ember - Polymorphic relationship - POST request not triggered

I'm facing an issue right now. Here's the scenario.

owner belongsTo pet

pet belongsTo owner

pet relationship can have 2 different types which are the dog and cat

The problem I'm facing right now is, the POST request doesn't triggered

const owner = this.get('owner');
const petDog = this.get('dog');

owner.set('pet', petDog);
owner.save(); // The POST request doesn't trigger