title: Many to Many relationship in emberfire
body: <p>I was wondering wether it is possible to create a many to many relationship in emberfire.
  I've got the following models:</p>
  
  <pre><code>//Employee
  name: DS.attr('string'),
  position: DS.attr('string'),
  accessoryPosition: DS.attr('string'),
  education: DS.attr('string'),
  experience: DS.attr('string'),
  imgUrl: DS.attr('string'),
  teachingIn: DS.attr('string'),
  subjects: DS.hasMany('subject', {async: true})
  
  //Subject
  name: DS.attr('string'),
  courseDescriptionUrl: DS.attr('string'),
  description: DS.attr('string'),
  examDescriptionUrl: DS.attr('string'),
  imgUrl: DS.attr('string'),
  sportsSubject: DS.attr('boolean'),
  outdoorSubject: DS.attr('boolean'),
  commonSubject: DS.attr('boolean'),
  teachers: DS.hasMany('employee', {async : true})
  </code></pre>
  
  <p>And then in my employee controller:</p>
  
  <pre><code>update(subjects) {
        this.get('model.employee').set('subjects',subjects);
        this.get('model.employee').save();
  
      }
  </code></pre>
  
  <p>But his only adds it to my /employees endpoint. Is there anyway to make the relationship mutual so to speak?</p>