Working with LoopBack objects

Overview

The primary LoopBack JavaScript objects are:

• App object
• Models
• Data sources

How to get a reference to these objects depends on where the code is (in a boot script, in a model JavaScript file /common/models/model.js, and so on) as well as which object you want to reference.

Getting the app object

Getting a reference to the app object is crucial, since from it you can obtain references to other objects such as models and data sources. You’ll typically want to get a handle on the app object in:

• Model scripts: /common/models/_modelName_.js (where modelName is the name of the model).
• Boot scripts in /server/boot

• Middleware (the ones you register in boot scripts and the ones in /server/server.js)

• Your own custom scripts

The app object provides context into various parts of a typical LB app.

From a boot script 

To get a reference to the app object in a boot script, pass it as the first parameter in the exported function.

Asynchronous boot script with a callback function:

module.exports = function(app, cb) { //app is injected by LoopBack
  //...
};

Synchronous boot script without a callback function:

module.exports = function(app) { //app is injected by loopback
  //...
};

As you can see from both examples, LoopBack provides the app object automatically as the first parameter in your boot scripts.

See Defining boot scripts for more information about boot scripts.

From middleware

LoopBack sets app object automatically in the request object in middleware (actually, under the hood, Express does it). You can access in server/server.js as follows:

...
app.use(function(req, res, next) {
  var app = req.app;
  //...
});
...

See Defining middleware for more information on middleware.

From a custom script

If you need a reference to app in your own custom scripts, simply require it (as in the models example):

var app = require('/server/server');
...

You simply require /server/server.js as you would any Node module.

From a model script

To get a handle on the app object in a model scaffolded by the Model generator, “require” it as you would any Node module:

var app = require('../../server/server'); //require `server.js` as in any node.js app

module.exports = function(Book) {
  //...
};

With models, there is a special case. From anywhere except /common/models/model.js, you can actually get a reference to app through a model using model.app.

For instance:

...
Book.app
...

However, the one caveat to this is that you cannot reference model.app in /common/models/model.js because this file does not add the app property until bootstrapping has finished. This means you cannot do the following in /common/models/model.js:

module.exports = function(Book) {
  Book.app... //won't work because `.app` has not been added to the Book object yet
});

However, you can get a reference to the app INSIDE remote methods, remote hooks, and model hooks because those are trigger after the application finishes loading (that is, after loopback.boot runs | after /server/server.js calls boot(..)).

This means you CAN do:

module.exports = function(Book) {
  Book.read(cb) {
    var app = Book.app;
    console.log(app.models...)
    cb();
  };
  Book.remoteMethod(
    'read',
    ...
  });
};

Of course, you can do the same in remote hooks and remote methods, but be aware of the load timing. Simply put, model.app will not be available until the application has completed bootstrapping, that is run boot() in /server/server.js. The idea here is to define our models before they are added to the application. Once the application finishes bootstrapping, you can then access a model’s app property. 

The easiest way of accessing the app object is via Model.on('attached') event.

module.exports = function(MyModel) {
  var app;
  MyModel.on('attached', function(a) {
    app = a;
    // perform any setup that requires the app object
  });
};

Working with the app object

The LoopBack app object is defined in the main script as follows:

var loopback = require('loopback');
var app = loopback();

The app object extends the Express app object; it inherits all of its properties and methods, as well as all the additional properties and methods of the LoopBack app object.

Working with model objects

Getting references to models

Once you get a handle on the app object, you can get a handle on to specific models via the models property on the app object.

module.exports = function(app) {
  var app = app.models.Book;
  //...
};

In your own custom script:

var app = require('/server/server');

Using model objects

For information on the basic model object, see Basic model object. For information on model object when the model is connected to a persistent data source, see Connected model object.

Getting a reference to an unrelated model

You can easily reference a related model with an expression such as MyModel.app.models.MyRelatedModel.  But this won’t work if there is no relation to the other model.  In this case, you need to get a reference to the app object using

require('../../server/server')

For example, suppose you want to reference the User model within an observer (hook), for example:

module.exports = function(MyModel) {
  var app = require('../../server/server');

  MyModel.observe('loaded', function( ctx, next) {
   var User = app.models.User;
   ...
   User.create(...);
   ...
});

Working with data source objects

Getting references to data sources

Similar to getting a handle on a model you first get a handle onto the app object, then you access the app.datasources property:

module.exports = function(app) {
  var dataSource = app.datasources.db; //db can be any registered datasource in `/server/datasources.json`
  ...
};

And in your own script:

var app = require('./server/server');
...
var datasource = app.datasources.db;
...

In middleware:

...
app.use(function(req, res, next) {
  var dataSource = app.datasources.db;
  ...
});
...

In models:

module.exports = function(Book) {
  Book.read = function() {
    var dataSource = Book.app.datasources.db;
  };
  Book.remoteMethod(
    'read',
     ...
  );
};

Be careful in models, because the following will not work:

module.exports = function(Book) {
  Book.app... //`Book` is not registered yet! This WON'T WORK.
};

Using data source objects