Here is what I suggested in PR: https://github.com/emberjs/data/pull/1751 ... https://github.com/pixelhandler/data/blob/serialize-has-many-key/packages/ember-data/lib/serializers/json_serializer.js#L382-L392 (with change for Ember.A#mapBy) 

```
serializeHasMany: function(record, json, relationship) {
    var key = relationship.key;
    var payloadKey = this.keyForRelationship ? this.keyForRelationship(key, "hasMany") : key;
    var relationshipType = RelationshipChange.determineRelationshipType(record.constructor, relationship);
    var relationshipTypes = ['manyToNone', 'manyToMany', 'manyToOne'];

    if (relationshipTypes.indexOf(relationshipType) > -1) {
      json[payloadKey] = Ember.A(record.get(key)).mapBy('id');
    }
  },
```