code: function(record, json, relationship) {
          var attr = relationship.key;
          var belongsTo = record.belongsTo(attr);
          var type, key;
  
          if (isNone(belongsTo)) {
            return;
          }
  
          type = this.keyForSnapshot(belongsTo);
          key = this.keyForRelationship(attr);
  
          if (!json.links) {
            json.links = json.relationships || {};
          }
          json.links[key] = belongsToLink(key, type, get(belongsTo, 'id'));
        }
docs: Use "links" key, remove support for polymorphic type