EmberJS action to add multiple user to a department. Not Working

show.js

    addMembers() {
      const { department, selected } = this.getProperties('department', 'selected');

      Ember.RSVP.all(
        selected.map((user) => {
          if(!Ember.isNone(user.get('department'))) {
            user.get('department').then((old) => {
              old.get('members').removeObject(user);
              old.save();
            });
          }
          department.get('members').pushObject(user);
          return user.save();
        })
      ).then(() => {
        department.save()
        this.set('selected', []);
      })
    },