New Twiddle

models.bar.js

import Model from "ember-data/model";
import attr from "ember-data/attr";

export default Model.extend({
  name: attr('string')
});

models.baz.js

import Model from "ember-data/model";
import attr from "ember-data/attr";

export default Model.extend({
  name: attr('string')
});

models.foo.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'),
  bar: belongsTo('bar', { inverse: null }),
  bazs: hasMany('baz')
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    $.mockjax({
      url: '/foos/1',
      responseText: {
        foo: {
          id: '1',
          name: 'I am foo',
          bar: '2'
        },
        bars: [{
          id: '2',
          name: 'I am bar'
        }]
      }
    });
    return this.get('store').findRecord('foo', 1);
  },
  afterModel(model) {
    let baz1 = this.get('store').createRecord('baz', { name: 'I am baz 1' });
    let baz2 = this.get('store').createRecord('baz', { name: 'I am baz 2' });
    model.get('bazs').pushObject(baz1);
    model.get('bazs').pushObject(baz2);
    
    model.save({
      adapterOptions: {
      	embedOrg: true
      }
    });
  }
});

serializers.bar.js

import RESTSerializer from 'ember-data/serializers/rest';

import DS from 'ember-data';

export default RESTSerializer.extend(DS.EmbeddedRecordsMixin);

serializers.baz.js

import RESTSerializer from 'ember-data/serializers/rest';

import DS from 'ember-data';

export default RESTSerializer.extend(DS.EmbeddedRecordsMixin);

serializers.foo.js

import DS from 'ember-data';
import RESTSerializer from 'ember-data/serializers/rest';

export default RESTSerializer.extend(DS.EmbeddedRecordsMixin, {
  attrs: {
		//bar: {
		//	serialize: 'records',
		//	deserialize: 'id'
		//},
		bazs: {
			serialize: 'records',
			deserialize: false
		}
	},
  
	serializeBelongsTo(snapshot, json, relationship) {
		if (relationship.key === 'bar' && snapshot.adapterOptions && snapshot.adapterOptions.embedOrg === true) {
			this._serializeEmbeddedBelongsTo(snapshot, json, relationship); 
		} else {
			this._super(snapshot, json, relationship);
		}
	}
});

templates.application.hbs

See JSON sent to mockjax in console...

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.7.0",
    "ember-data": "2.7.0",
    "ember-template-compiler": "2.7.0",
    "jquery-mockjax": "https://cdnjs.cloudflare.com/ajax/libs/jquery-mockjax/1.6.2/jquery.mockjax.js"
  },
  "addons": {}
}