executor: remove direct reference to loopback

Modify the executor to access the loopback object via `app.loopback`.

Fall back to `require('loopback')` only when `app.loopback` is not set
(loopback versions before 1.9).

lib/executor.js

@@ -1,6 +1,5 @@
 var assert = require('assert');
 var _ = require('underscore');
-var loopback = require('loopback');
 var semver = require('semver');
 var debug = require('debug')('loopback:boot:executor');
 
@@ -14,6 +13,7 @@ var debug = require('debug')('loopback:boot:executor');
  */
 
 module.exports = function execute(app, instructions) {
+  patchAppLoopback(app);
   assertLoopBackVersion(app);
 
   setHost(app, instructions);
@@ -23,25 +23,38 @@ module.exports = function execute(app, instructions) {
 
   setupDataSources(app, instructions);
   setupModels(app, instructions);
-  autoAttach();
+  autoAttach(app);
 
   runBootScripts(app, instructions);
 
   enableAnonymousSwagger(app, instructions);
 };
 
+function patchAppLoopback(app) {
+  if (app.loopback) return;
+  // app.loopback was introduced in 1.9.0
+  // patch the app object to make loopback-boot work with older versions too
+  try {
+    app.loopback = require('loopback');
+  } catch(err) {
+    if (err.code === 'MODULE_NOT_FOUND') {
+      console.error(
+          'When using loopback-boot with loopback <1.9, '+
+          'the loopback module must be available for `require(\'loopback\')`.');
+    }
+    throw err;
+  }
+}
+
 function assertLoopBackVersion(app) {
   var RANGE = '1.x || 2.x';
 
-  // app.loopback was introduced in 1.9.0
+  var loopback = app.loopback;
-  var loopback = app.loopback || {};
+  if (!semver.satisfies(loopback.version || '1.0.0', RANGE)) {
-  var version = loopback.version || '1.0.0';
-
-  if (!semver.satisfies(version, RANGE)) {
     throw new Error(
         'The `app` is powered by an incompatible loopback version %s. ' +
         'Supported versions: %s',
-        loopback.version || '<1.9',
+        loopback.version || '(unknown)',
       RANGE);
   }
 }
@@ -134,14 +147,14 @@ function runScripts(app, list) {
   if (!list || !list.length) return;
   list.forEach(function(filepath) {
     var exports = tryRequire(filepath);
-    if (isFunctionNotModelCtor(exports))
+    if (isFunctionNotModelCtor(exports, app.loopback.Model))
       exports(app);
   });
 }
 
-function isFunctionNotModelCtor(fn) {
+function isFunctionNotModelCtor(fn, Model) {
   return typeof fn === 'function' &&
-    !(fn.prototype instanceof loopback.Model);
+    !(fn.prototype instanceof Model);
 }
 
 function tryRequire(modulePath) {
@@ -158,9 +171,9 @@ function tryRequire(modulePath) {
 }
 
 // Deprecated, will be removed soon
-function autoAttach() {
+function autoAttach(app) {
   try {
-    loopback.autoAttach();
+    app.loopback.autoAttach();
   } catch(e) {
     if(e.name === 'AssertionError') {
       console.warn(e);