Serialize Changed Relationships

adapters.application\.js

export default class AppAdapter {
  constructor(args) { Object.assign(this, args); }
  static create(args) { return new this(args); }
  
  updateRecord(store, schema, snapshot) {
    const posts = snapshot.record.hasMany('posts').hasManyRelationship;
    const remote = posts.canonicalMembers.copy();
    const local = posts.members.copy();
    console.log({ remote, local });
    return;
  }
}

controllers.application.js

import Ember from 'ember';
import { inject as service } from '@ember/service';

export default Ember.Controller.extend({
});

helpers.v\.js

import { helper } from '@ember/component/helper';

export default helper(function v(params/*, hash*/) {
  switch (params[0]) {
    case undefined:
      return '[[UNDEFINED]]';
    case null:
      return '[[NULL]]';
    default:
      return params[0];
  }
});

models.author.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  name: attr('string'),
  posts: hasMany('post', { inverse: 'author', async: false }),
});

models.post.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  title: attr('string'),
  author: belongsTo('author', { inverse: 'posts', async: false }),
});

routes.application.js

import Ember from 'ember';

function makePost(id, addAuthor = true) {
  return {
    type: 'post',
    id,
    attributes: { title: `post number ${id}` },
    relationships: {
      author: { data: addAuthor ? { type: 'author', id: '1' } : null }
    }
  }
}

export default Ember.Route.extend({
  async model() {
    const { store } = this;
    
    const author = store.push({
      data: {
        type: 'author',
        id: '1',
        attributes: { name: 'Chris' },
        relationships: {
          posts: {
            data: [
              { type: 'post', id: '1' },
              { type: 'post', id: '2' },
              { type: 'post', id: '3' },
              { type: 'post', id: '4' },
              { type: 'post', id: '5' },
            ]
          }
        }
      },
      included: [
         makePost('1'),
         makePost('2'),
         makePost('3'),
         makePost('4'),
         makePost('5'),
      ]
    });
    
    const newPost = store.createRecord('post', {
      title: 'new post',
    });
    const otherPost = store.push({ data: makePost('6', false) });
    const post4 = store.peekRecord('post', '4');
    
    // remove a post
    author.posts.removeObject(post4);
    
    // add new post
    author.posts.pushObject(newPost);
    
    // update with a new post
    author.posts.pushObject(otherPost);
    
    await author.save();
    
    return author;
  }
});

templates.application.hbs

<h3>posts</h3>

<ul>
  {{#each this.model.posts as |post|}}
    <li>{{post.id}} {{post.title}}</li>
  {{/each}}
</ul>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.16.0"
  }
}