js/libs/ember-data.js
function(record, json, relationship) {
    var key = relationship.key;

    var belongsTo = get(record, key);

    key = this.keyForRelationship ? this.keyForRelationship(key, "belongsTo") : key;

    if (isNone(belongsTo)) {
      json[key] = belongsTo;
    } else {
      json[key] = get(belongsTo, 'id');
    }

    if (relationship.options.polymorphic) {
      this.serializePolymorphicType(record, json, relationship);
    }
  }
function(record, json, relationship) {
    var key = relationship.key;

    var belongsTo = get(record, key);

    key = this.keyForRelationship ? this.keyForRelationship(key, "belongsTo") : key;

    if (isNone(belongsTo)) {
      json[key] = belongsTo;
    } else {
      json[key] = get(belongsTo, 'id');
    }

    if (relationship.options.polymorphic) {
      this.serializePolymorphicType(record, json, relationship);
    }
  }
`serializeBelongsTo` can be used to customize how `DS.belongsTo`
properties are serialized.

Example

```javascript
App.PostSerializer = DS.JSONSerializer.extend({
serializeBelongsTo: function(record, json, relationship) {
var key = relationship.key;

var belongsTo = get(record, key);

key = this.keyForRelationship ? this.keyForRelationship(key, "belongsTo") : key;

json[key] = Ember.isNone(belongsTo) ? belongsTo : belongsTo.toJSON();
}
});
```

@method serializeBelongsTo
@param {DS.Model} record
@param {Object} json
@param {Object} relationship