Only validate dataSource when defined (Fixes #482)

lib/application.js

@@ -390,13 +390,15 @@ function configureModel(ModelCtor, config, app) {
 
   var dataSource = config.dataSource;
 
-  if(typeof dataSource === 'string') {
-    dataSource = app.dataSources[dataSource];
-  }
+  if(dataSource) {
+    if(typeof dataSource === 'string') {
+      dataSource = app.dataSources[dataSource];
+    }
 
-  assert(dataSource instanceof DataSource,
-      ModelCtor.modelName + ' is referencing a dataSource that does not exist: "' +
-        config.dataSource +'"');
+    assert(dataSource instanceof DataSource,
+        ModelCtor.modelName + ' is referencing a dataSource that does not exist: "' +
+          config.dataSource +'"');
+  }
 
   config = extend({}, config);
   config.dataSource = dataSource;

test/app.test.js

@@ -69,6 +69,10 @@ describe('app', function() {
         request(app).get('/colors').expect(200, done);
       });
     });
+
+    it('should not require dataSource', function() {
+      app.model('MyTestModel', {});
+    });
   });
 
   describe('app.model(name, config)', function () {