fix undefined id in relationship link url

Author: JonathanPrince

lib/serializer.js

@@ -114,12 +114,10 @@ function parseRelations (data, relations, options) {
 
   _.each(relations, function (relation, name) {
 
-    var pkName = relation.keyFrom;
     var fkName = relation.keyTo;
 
-    // If relation is belongsTo then pk and fk are the other way around
+    // If relation is belongsTo then fk is the other way around
     if (relation.type === 'belongsTo' || relation.type === 'referencesMany') {
-      pkName = relation.keyTo;
       fkName = relation.keyFrom;
     }
 
@@ -134,7 +132,7 @@ function parseRelations (data, relations, options) {
       return false;
     }
 
-    var pk = data[pkName];
+    var pk = data[options.primaryKeyField];
     var fk = data[fkName];
 
     var fromType = utils.pluralForModel(relation.modelFrom);