Skip definitions of built-in loopback models

LoopBack built-in models are special: they follow the loopback-boot
structure and provide `common/models/{name}.json` files, but they are
also automatically loaded (created) by loopback.

This change modifies `executor` to recognize built-in models and do not
redefine them.

lib/executor.js

@@ -156,6 +156,10 @@ function defineModels(app, instructions) {
         throw new Error('Cannot configure unknown model ' + name);
       }
       debug('Configuring existing model %s', name);
+    } else if (isBuiltinLoopBackModel(app, data)) {
+      model = app.loopback[name];
+      assert(model, 'app.loopback[model] should have been defined');
+      debug('Configuring built-in LoopBack model %s', name);
     } else {
       debug('Creating new model %s %j', name, data.definition);
       model = app.loopback.createModel(data.definition);
@@ -176,6 +180,15 @@ function defineModels(app, instructions) {
   });
 }
 
+function isBuiltinLoopBackModel(app, data) {
+  // 1. Built-in models are exposed on the loopback object
+  if (!app.loopback[data.name]) return false;
+
+  // 2. Built-in models have a script file `loopback/{facet}/models/{name}.js`
+  return data.sourceFile &&
+    /node_modules\/loopback\/[^\/]+\/models\/[^\/]+\.js$/.test(data.sourceFile);
+}
+
 function forEachKeyedObject(obj, fn) {
   if(typeof obj !== 'object') return;
 

test/executor.test.js

@@ -160,6 +160,22 @@ describe('executor', function() {
     expect(actual).to.equal('not attached');
   });
 
+  it('skips definition of already defined LoopBack models', function() {
+    var builtinModel = {
+      name: 'User',
+      definition: fs.readJsonFileSync(
+        require.resolve('loopback/common/models/user.json')
+      ),
+      config: { dataSource: 'db' },
+      sourceFile: require.resolve('loopback/common/models/user.js')
+    };
+    builtinModel.definition.redefined = true;
+
+    boot.execute(app, someInstructions({ models: [ builtinModel ] }));
+
+    expect(app.models.User.settings.redefined, 'redefined').to.not.equal(true);
+  });
+
   describe('with boot and models files', function() {
     beforeEach(function() {
       process.bootFlags = process.bootFlags || [];

test/fixtures/browser-app/model-config.json

@@ -1,4 +1,10 @@
 {
+  "_meta": {
+    "sources": [
+      "./models",
+      "../../../node_modules/loopback/common/models"
+    ]
+  },
   "Customer": {
     "dataSource": "db"
   }