Ember - Many to many relationship data not being updated

I have a model for a speaker as follows:

import attr from 'ember-data/attr';
import ModelBase from 'open-event-frontend/models/base';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default ModelBase.extend({

  /**
   * Attributes
   */

  name               : attr('string'),
  email              : attr('string'),
  photoUrl           : attr('string'),
  thumbnailImageUrl  : attr('string'),
  shortBiography     : attr('string'),
  longBiography      : attr('string'),
  speakingExperience : attr('string'),
  mobile             : attr('string'),
  location           : attr('string'),
  website            : attr('string'),
  twitter            : attr('string'),
  facebook           : attr('string'),
  github             : attr('string'),
  linkedin           : attr('string'),
  organisation       : attr('string'),
  isFeatured         : attr('boolean', { default: false }),
  position           : attr('string'),
  country            : attr('string'),
  city               : attr('string'),
  gender             : attr('string'),
  heardFrom          : attr('string'),

  /**
   * Relationships
   */

  user     : belongsTo('user'),
  event    : belongsTo('event'),
  sessions : hasMany('session')

});

And a model for a session as follows:

import attr from 'ember-data/attr';
import moment from 'moment';
import ModelBase from 'open-event-frontend/models/base';
import { belongsTo, hasMany } from 'ember-data/relationships';
import { computedDateTimeSplit } from 'open-event-frontend/utils/computed-helpers';

const detectedTimezone = moment.tz.guess();

export default ModelBase.extend({
  title         : attr('string'),
  subtitle      : attr('string'),
  startsAt      : attr('moment', { defaultValue: () => moment.tz(detectedTimezone).add(1, 'months').startOf('day') }),
  endsAt        : attr('moment', { defaultValue: () => moment.tz(detectedTimezone).add(1, 'months').hour(17).minute(0) }),
  shortAbstract : attr('string'),
  longAbstract  : attr('string'),
  language      : attr('string'),
  level         : attr('string'),
  comments      : attr('string'),
  state         : attr('string'),
  slidesUrl     : attr('string'),
  videoUrl      : attr('string'),
  audioUrl      : attr('string'),
  signupUrl     : attr('string'),
  sendEmail     : attr('boolean'),

  isMailSent: attr('boolean', { defaultValue: false }),

  createdAt      : attr('string'),
  deletedAt      : attr('string'),
  submittedAt    : attr('string', { defaultValue: () => moment() }),
  lastModifiedAt : attr('string'),
  sessionType    : belongsTo('session-type'),
  microlocation  : belongsTo('microlocation'),
  track          : belongsTo('track'),
  speakers       : hasMany('speaker'),
  event          : belongsTo('event'), // temporary
  creator        : belongsTo('user'),

  startAtDate : computedDateTimeSplit.bind(this)('startsAt', 'date'),
  startAtTime : computedDateTimeSplit.bind(this)('startsAt', 'time'),
  endsAtDate  : computedDateTimeSplit.bind(this)('endsAt', 'date'),
  endsAtTime  : computedDateTimeSplit.bind(this)('endsAt', 'time')
});

As is clear, session and speaker share a many-to-many relationship. So I am adding the session to the speaker and then saving them. Both the records are successfully created on the server but the link is not established. I have tested the server endpoint with postman and it works fine. So, I guess I am missing something here.

This is the controller code:

import Controller from '@ember/controller';
export default Controller.extend({
  actions: {
    save() {
      this.set('isLoading', true);
      this.get('model.speaker.sessions').pushObject(this.get('model.session'));

      this.get('model.session').save()
        .then(() => {
          this.get('model.speaker').save()
            .then(() => {
              this.get('notify').success(this.get('l10n').t('Your session has been saved'));
              this.transitionToRoute('events.view.sessions', this.get('model.event.id'));
            })
            .catch(() => {
              this.get('notify').error(this.get('l10n').t('Oops something went wrong. Please try again'));
            });
        })
        .catch(() => {
          this.get('notify').error(this.get('l10n').t('Oops something went wrong. Please try again'));
        })
        .finally(() => {
          this.set('isLoading', false);
        });
    }
  }
});