js/libs/ember-data.js
function(record, json, relationship) {
    var key = relationship.key,
        belongsTo = get(record, key);
    key = this.keyForAttribute(key);
    json[key + "_type"] = Ember.String.capitalize(belongsTo.constructor.typeKey);
  }
function(record, json, relationship) {
    var key = relationship.key,
        belongsTo = get(record, key);
    key = this.keyForAttribute(key);
    json[key + "_type"] = Ember.String.capitalize(belongsTo.constructor.typeKey);
  }
Serializes a polymorphic type as a fully capitalized model name.

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