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#LoopBack model relations example app The purpose of this example is to demonstrate model relations in LoopBack.

##Contents

##Overview We will create a web application to demonstrate model relations. The main page will consist of various links that allow us to query and filter data through an exposed REST API.

##Prerequisites This guide assumes you have working knowledge of:

You should also have the following installed:

##Example ###1. Getting started Let's begin by scaffolding the application:

slc loopback

You should see:

...
[?] Enter a directory name where to create the project: (.)

Enter loopback-example-relation as the project name (we'll refer to the generated directory as the project root from hereon). Finish the creation process by following the prompts.

###2. Create the models We will be using an in-memory database to hold our data. Create a model named Customer by running:

cd loopback-example-relation
slc loopback:model Customer

You should see:

[?] Enter the model name: Customer
[?] Select the data-source to attach Customer to: db (memory)
[?] Expose Customer via the REST API? Yes
[?] Custom plural form (used to build REST URL):
Let's add some Customer properties now.

Enter an empty property name when done.
[?] Property name: name
   invoke   loopback:property
[?] Property type: string
[?] Required? No

Let's add another Customer property.
Enter an empty property name when done.
[?] Property name: age
   invoke   loopback:property
[?] Property type: number
[?] Required? No

Let's add another Customer property.
Enter an empty property name when done.
[?] Property name: #leave blank, press enter

Follow the prompts to finish creating the model. Repeat for Review and Order using the following properties:

  • Review
    • product:string
    • star:number
  • Order
    • description:string
    • total:number

You should see customer.json, order.json and review.json in common/models when you're done.

###3. Create the front-end Let's create a front-end to make it easier to analyze our data. To install EJS, run the following from the project root:

npm install --save ejs

Then configure the application view engine by modifying server/server.js to look like:

...
// -- Mount static files here--
...
app.set('view engine', 'html');
app.engine('html', require('ejs').renderFile);
app.set('json spaces', 2); //pretty print results for easier viewing later
...

Next, modify server/boot/root.js to look like:

module.exports = function(server) {
  var router = server.loopback.Router();
  router.get('/', function(req, res) {
    res.render('index');
  });
  server.use(router);
};

Finally, create the views directory by running:

mkdir -p server/views

Inside the views directory, create index.html with the following contents:

<DOCTYPE html>
<html>
  <head>
    <title>loopback-example-relation</title>
  </head>
  <body>
    <h1>loopback-example-relation</h1>
    <p>
      <a href="/explorer">API Explorer</a>
    </p>
    <h2>API</h2>
    <ul>
      <li><a href='/api/customers'>/api/customers</a>
      <li><a href='/api/customers?filter[fields][name]=true'>/api/customers?filter[fields][name]=true</a>
      <li><a href='/api/customers/1'>/api/customers/1</a>
      <li><a href='/api/customers/youngFolks'>/api/customers/youngFolks</a>
      <li><a href='/api/customers/1/reviews'>/api/customers/1/reviews</a>
      <li><a href='/api/customers/1/orders'>/api/customers/1/orders</a>
      <li><a href='/api/customers?filter[include]=reviews'>/api/customers?filter[include]=reviews</a>
      <li><a href='/api/customers?filter[include][reviews]=author'>/api/customers?filter[include][reviews]=author</a>
      <li><a href='/api/customers?filter[include][reviews]=author&filter[where][age]=21'>/api/customers?filter[include][reviews]=author&filter[where][age]=21</a>
      <li><a href='/api/customers?filter[include][reviews]=author&filter[limit]=2'>/api/customers?filter[include][reviews]=author&filter[limit]=2</a>
      <li><a href='/api/customers?filter[include]=reviews&filter[include]=orders'>/api/customers?filter[include]=reviews&filter[include]=orders</a>
    </ul>
  </body>
</html>

You can view what we have so far by executing slc run server from the project root and browsing to localhost:3000. Click on API Explorer and you will notice that the models we created from step 2 are there.

You may also notice some of the API endpoints return empty arrays or errors. It's because the database is empty. In addition, we need to define model relations for some of the API endpoints to work. Don't fret, we'll get to all that very soon!

###4. Add sample data In server/boot, create a script named create-customers.js with the following contents:

var customers = [
  {name: 'Customer A', age: 21},
  {name: 'Customer B', age: 22},
  {name: 'Customer C', age: 23},
  {name: 'Customer D', age: 24},
  {name: 'Customer E', age: 25}
];

module.exports = function(server) {
  var dataSource = server.dataSources.db;
  dataSource.automigrate('customer', function(er) {
    if (er) throw er;
    var Model = server.models.Customer;
    //create sample data
    var count = customers.length;
    customers.forEach(function(customer) {
      Model.create(customer, function(er, result) {
        if (er) return;
        console.log('Record created:', result);
        count--;
        if (count === 0) {
          console.log('done');
          dataSource.disconnect();
        }
      });
    });
    //define a custom scope
    Model.scope('youngFolks', {where: {age: {lte: 22 }}});
  });
};

Create two more scripts, create-reviews.js and create-orders.js in server/boot. This sample data will be automatically loaded when you start the application.

automigrate() recreates the database table/index if it already exists. In other words, existing tables will be dropped and ALL EXISTING DATA WILL BE LOST. For more information, see the documentation.

Model.scope... is only in create-customers.js.

###5. Create model relations From the project root, run:

slc loopback:relation

Follow the prompts and create the following relationships:

  • Customer
    • has many
      • Review
        • property name for the relation: reviews
        • custom foreign key: authorId
      • Order
        • property name for the relation: orders
        • custom foreign key: customerId
  • Review
    • belongs to
      • Customer
        • property name for the relation: author
        • custom foreign key: authorId
  • Order
    • belongs to
      • Customer

For any item without property name for the relation or custom foreign key, just use the defaults. LoopBack will derive these values automatically when you don't specify one.

When you're done, your common/models/customer.json should look like:

{
  "name": "Customer",
  "base": "PersistedModel",
  "properties": {
    "name": {
      "type": "string"
    },
    "age": {
      "type": "number"
    }
  },
  "validations": [],
  "relations": {
    "reviews": {
      "type": "hasMany",
      "model": "Review",
      "foreignKey": "authorId"
    },
    "orders": {
      "type": "hasMany",
      "model": "Order",
      "foreignKey": "customerId"
    }
  },
  "acls": [],
  "methods": []
}

common/models/reviews.json should look like:

{
  "name": "Review",
  "base": "PersistedModel",
  "properties": {
    "product": {
      "type": "string"
    },
    "star": {
      "type": "number"
    }
  },
  "validations": [],
  "relations": {
    "author": {
      "type": "belongsTo",
      "model": "Customer",
      "foreignKey": "authorId"
    }
  },
  "acls": [],
  "methods": []
}

and common/models/order.json should look like:

{
  "name": "Order",
  "base": "PersistedModel",
  "properties": {
    "description": {
      "type": "string"
    },
    "total": {
      "type": "number"
    }
  },
  "validations": [],
  "relations": {
    "customer": {
      "type": "belongsTo",
      "model": "Customer",
      "foreignKey": ""
    }
  },
  "acls": [],
  "methods": []
}

You should be creating four relations in total: Customer has many Reviews, Customer has many Orders, Review belongs to Customer and Order belongs to Customer.

###6. Try the API Restart application (slc run server in case you forgot) and browse to localhost:3000. Each endpoint should be working properly now that we've defined the model relations. See the following endpoint descriptions:

###7. Conclusion That's it! You've successfully created an application with models formed into a complex data graph! For a deeper dive into relations, see this blog entry (the examples were written for LoopBack 1.x, but the concepts still apply). If you have further questions, please refer to the LoopBack documentation.

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LoopBack example demonstrating basic model relations

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