project/app/serializers/useful.js

import DS from 'ember-data';
import Ember from 'ember';

export default DS.JSONAPISerializer.extend({
  serializeBelongsTo: function(record, json, relationship) {
    var key = relationship.key;
    var belongsTo = Ember.get(record, key);
    key = this.keyForRelationship ? this.keyForRelationship(key, 'belongsTo') : key;

    if (relationship.options.embedded === 'always') {
      json[key] = belongsTo.serialize();
    }    else {
      return this._super(record, json, relationship);
    }
  },
  serializeHasMany: function(record, json, relationship) {
    var key = relationship.key;
    var hasMany = Ember.get(record, key);
    var relationshipType = DS.RelationshipChange.determineRelationshipType(
      record.constructor,
      relationship);

    if (relationship.options.embedded === 'always') {
      if (hasMany && relationshipType === 'manyToNone' || relationshipType === 'manyToMany' ||
          relationshipType === 'manyToOne') {

        json[key] = [];
        hasMany.forEach(function(item) {
          json[key].push(item.serialize());
        });
      }

    }    else {
      return this._super(record, json, relationship);
    }
  }
});