hasMany relationship not updating in the database

When I save a Block (which belongs to a Cube), the belongsTo relationship is saved but the hasMany relationship for the Cube is not. After creating the Block, no PUT request is being sent to the server to update the Cube.

Block Model:

export default DS.Model.extend({
  name: DS.attr(),
  description: DS.attr(),
  cube: DS.belongsTo('cube')
});

Cube Model:

export default DS.Model.extend({
  name: DS.attr(),
  description: DS.attr(),
  blocks: DS.hasMany('block', { async: true })
});

Saving the Block:

  var name = this.get('name');
  var description = this.get('description');
  var cube = this.store.peekRecord('cube', this.model.id);

  if (!name) {return;}

  this.store.createRecord('block', {
    name: name,
    description: description,
    cube: cube
  })
  .save()
  .then(function() {
    this.resetForm();
    var name = slugify(this.model.get('name'));
    this.transitionToRoute('cubes.show', this.model.get('id'), name);
  }
  .bind(this));