enforcing uniqueness in loopback4 model's properties

Question

I started to run a simple hello world project using lb4. I created a basic mongodb datasource, and a repository and a simple model (username, password and id) for users with cli command provided by loopback. I also created a user controller, with a built in crud choice provided by loopback.
as you may know, this is a very basic setup but if anyone needs details, I will provide, just comment.
the problem is that, when I try to make new users using explorer, I encounter with duplicate same username models. means that:

{
"_id" : ObjectId("5def4581f7f9d508b0da2d4c"),
"username" : "string",
"password" : "string"
}

and:

{
"_id" : ObjectId("5def4584f7f9d508b0da2d4d"),
"username" : "string",
"password" : "string"
}

the client should enter unique username for signup. that's obvious.
how can I specify uniqueness of a property of a model in loopback 4 (without declaring it as id)?

Answer 1

Due to this issue on github, lb4 already checks for duplicates inside of its context using indexes and 11000 error code of mongoDB (I am talking about shopping example, not a simple lb4 app so BE AWARE! If you want you can implement uniqueness using description below). What I forgot to do was database migration 🐱‍🏍.
you can do database migration with this command:
npm run migrate
so the complete answer should be like this:
1- inside the model add this: (change uniqueEmail and email to property name you want to be unique)

@model({
    settings: {
        indexes: {
          uniqueEmail: {
            keys: {
              email: 1,
            },
            options: {
              unique: true,
            },
         },
    },
  },
})

2- check inside of the controller, the endpoint you want to check uniqueness, and add a try catch to catch error of uniqueness which came from your mongoDB datasource:

try {
  // In my case create the new user, but can be anything you want
  const savedUser = await this.userRepository.create(
    _.omit(newUserRequest, 'password'),
  );

  // do stuff and return

} catch (error) {
  // MongoError 11000 duplicate key
  if (error.code === 11000 && error.errmsg.includes('index: uniqueEmail')) {
    throw new HttpErrors.Conflict('Email value is already taken');
  } else {
    throw error;
  }
}

3- run npm run migrate

This answer now is a general answer which can be used for any property of any model. Hope this helps.