How to rollback uncommited relationship after using pushObject?

Given a tag and post ManyToMany relationship defined as follow:

post.js

import Ember from 'ember';
import Model from 'ember-data/model';
import { hasMany } from 'ember-data/relationships';

export default Model.extend({
  tags: hasMany('tag')
}

tag.js

import Ember from 'ember';
import Model from 'ember-data/model';
import { hasMany } from 'ember-data/relationships';

export default Model.extend({
  posts: hasMany('post')
}

When adding tags to post using pushObject

let aPost = this.store.findRecord('post', 1),
    aTag  = this.store.findRecord('tag', 1);

aPost.get('tags').pushObject(aTag); // aPost is not commited yet

How to restore aPost.tags to its initial state?