Auto-configure models required by `app.enableAuth`

Modify `app.enableAuth` to automaticaly setup all required models
that are not attached to the app nor a datasource.

Users wishing to use this option must provide the name of the
data-source to use for these models.

Example usage:

    var app = loopback();
    app.dataSource('db', { connector: 'memory' });
    app.enableAuth({ dataSource: 'db' });

    app.use(loopback.rest());
    app.listen(3000);

lib/application.js

@@ -291,10 +291,37 @@ app.dataSources = app.datasources = {};
  * Enable app wide authentication.
  */
 
-app.enableAuth = function() {
+app.enableAuth = function(options) {
+  var AUTH_MODELS = ['User', 'AccessToken', 'ACL', 'Role', 'RoleMapping'];
+
   var remotes = this.remotes();
   var app = this;
 
+  if (options && options.dataSource) {
+    var appModels = app.registry.modelBuilder.models;
+    AUTH_MODELS.forEach(function(m) {
+      var Model = app.registry.findModel(m);
+      if (!Model) {
+        throw new Error(
+          'Authentication requires model ' + m + ' to be defined.');
+      }
+
+      if (m.dataSource || m.app) return;
+
+      for (var name in appModels) {
+        var candidate = appModels[name];
+        var isSubclass = candidate.prototype instanceof Model;
+        var isAttached = !!candidate.dataSource || !!candidate.app;
+        if (isSubclass && isAttached) return;
+      }
+
+      app.model(Model, {
+        dataSource: options.dataSource,
+        public: m === 'User'
+      });
+    });
+  }
+
   remotes.authorization = function(ctx, next) {
     var method = ctx.method;
     var req = ctx.req;

test/app.test.js

@@ -796,6 +796,34 @@ describe('app', function() {
       app.enableAuth();
       expect(app.isAuthEnabled).to.equal(true);
     });
+
+    it('auto-configures required models to provided dataSource', function() {
+      var AUTH_MODELS = ['User', 'ACL', 'AccessToken', 'Role', 'RoleMapping'];
+      var app = loopback({ localRegistry: true, loadBuiltinModels: true });
+      require('../lib/builtin-models')(app.registry);
+      var db = app.dataSource('db', { connector: 'memory' });
+
+      app.enableAuth({ dataSource: 'db' });
+
+      expect(Object.keys(app.models)).to.include.members(AUTH_MODELS);
+
+      AUTH_MODELS.forEach(function(m) {
+        var Model = app.models[m];
+        expect(Model.dataSource, m + '.dataSource').to.equal(db);
+        expect(Model.shared, m + '.shared').to.equal(m === 'User');
+      });
+    });
+
+    it('detects already configured subclass of a required model', function() {
+      var app = loopback({ localRegistry: true, loadBuiltinModels: true });
+      var db = app.dataSource('db', { connector: 'memory' });
+      var Customer = app.registry.createModel('Customer', {}, { base: 'User' });
+      app.model(Customer, { dataSource: 'db' });
+
+      app.enableAuth({ dataSource: 'db' });
+
+      expect(Object.keys(app.models)).to.not.include('User');
+    });
   });
 
   describe.onServer('app.get(\'/\', loopback.status())', function() {