js/libs/ember-data.js
function(record, json, relationship) {
    var key = relationship.key,
        belongsTo = get(record, key);
    key = this.keyForAttribute ? this.keyForAttribute(key) : key;
    json[key + "Type"] = belongsTo.constructor.typeKey;
  }
function(record, json, relationship) {
    var key = relationship.key,
        belongsTo = get(record, key);
    key = this.keyForAttribute ? this.keyForAttribute(key) : key;
    json[key + "Type"] = belongsTo.constructor.typeKey;
  }
You can use this method to customize how polymorphic objects are serialized.
By default the JSON Serializer creates the key by appending `Type` to
the attribute and value from the model's camelcased model name.

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