Connecting to MongoDB

The following tutorial introduces how to set up MongoDB as the data source of LoopBack 4 applications with LoopBack MongoDB connector.

Prerequisites

Before starting this tutorial, make sure you have the following installed:

• Node.js version 10 or higher
• LoopBack 4 CLI; see Getting Started with LoopBack 4

Tutorial - MongoDB

1. Create a new LoopBack 4 app

Let’s use the LB4 CLI lb4 app to create a LoopBack 4 application called MyApp:

$ lb4 app
? Project name: my-app
? Project description: MongoDB connector tutorial
? Project root directory: my-app
? Application class name: MyAppApplication
? Select features to enable in the project (Press <space> to select, <a> to togg
le all, <i> to invert selection)
❯◉ Enable eslint: add a linter with pre-configured lint rules
 ◉ Enable prettier: install prettier to format code conforming to rules
 ◉ Enable mocha: install mocha to run tests
 ◉ Enable loopbackBuild: use @loopback/build helpers (e.g. lb-eslint)
 ◉ Enable vscode: add VSCode config files
 ◉ Enable docker: include Dockerfile and .dockerignore
 ◉ Enable repositories: include repository imports and RepositoryMixin
(Move up and down to reveal more choices)

2. Create models

Let’s create a simple model User. To keep the tutorial short, the prompts of lb4 model are skipped:

// imports
@model()
export class User extends Entity {
  @property({
    type: 'number',
    id: true,
    generated: true,
  })
  id?: number;

  @property({
    type: 'string',
  })
  name?: string;

  @property({
    type: 'boolean',
    required: true,
  })
  hasAccount: boolean;

  constructor(data?: Partial<User>) {
    super(data);
  }
}

3. Create a data source

Next, let’s create a DataSource db using the MongoDB connector by the prompts below:

$ lb4 datasource
? Datasource name: db
? Select the connector for db:
  ...
  Redis key-value connector (supported by StrongLoop)
❯ MongoDB (supported by StrongLoop)
  MySQL (supported by StrongLoop)
  ...
? Connection String url to override other settings (eg: mongodb://username:passw
ord@hostname:port/database):
? host: localhost
? port: 27017
? user:
? password: [hidden]
? database: demo
? Feature supported by MongoDB v3.1.0 and above: Yes

Datasource Db was created in src/datasources/

Under src/datasources/db.datasource.ts, we can find the DbDataSource class and the config we just set:

const config = {
  name: 'db',
  connector: 'mongodb',
  url: '',
  host: 'localhost',
  port: 27017,
  user: '',
  password: '',
  database: 'demo',
};

4. Create repositories

A Repository is an artifact that ties the model and the datasource. We will need to create the repository for the User class to access the database. The steps of creating UserRepository by running lb4 repository are skipped here:

// imports
export class UserRepository extends DefaultCrudRepository<
  User,
  typeof User.prototype.id,
  UserRelations
> {
  constructor(@inject('datasources.db') dataSource: DbDataSource) {
    super(User, dataSource);
  }
}

5. Create endpoints and view data using API Explorer

Once we built a controller with lb4 controller to handle requests:

$ lb4 controller
? Controller class name: user
Controller User will be created in src/controllers/user.controller.ts

? What kind of controller would you like to generate? REST Controller with CRUD
functions
? What is the name of the model to use with this CRUD repository? User
? What is the name of your CRUD repository? UserRepository
? What is the name of ID property? id
? What is the type of your ID? number
? Is the id omitted when creating a new instance? Yes
? What is the base HTTP path name of the CRUD operations? /users

Notice that the id is omitted in the request here because it is autogenerated.

From the project root, start the app:

$ npm start

We can verify what we just created with API Explorer http://localhost:3000/explorer/.

Data Mapping Properties

If you’d like to use different names for the collection/fields and the model/properties, it can be achieved by configuring the model definition/property definition. Take the User model as an example, if we’d like to name the collection as MY_USER in the database and also use uppercase for properties name and hasAccount, the following settings would allow us to do so:

@model({
  settings: {
    // add it to the model definition
    mongodb: {collection: 'MY_USER'},
  },
})
export class User extends Entity {
  @property({
    type: 'string',
    id: true,
    generated: true,
  })
  id: string;

  @property({
    type: 'string',
    // add it to the property definition
    mongodb: {
      fieldName: 'NAME',
    },
  })
  name?: string;

  @property({
    type: 'boolean',
    required: true,
    // add it to the property definition
    mongodb: {
      fieldName: 'HASACCOUNT',
    },
  })
  hasAccount: boolean;
}

Handling ObjectId

MongoDB uses ObjectId for its primary key, which is an object instead of a string. In queries, string values must be cast to ObjectID, otherwise they are not considered as the same value. Therefore, you might want to specify the data type of properties to enforce ObjectId coercion. Such coercion would make sure the property value converts from ObjectId-like string to ObjectId when it accesses to the database and converts ObjectId to ObjectId-like string when the app gets back the value.

Please check section Handling ObjectId for details.