[CLI] Generate DataSource

[dhmlau]

Description / Feature proposal

CLI command to generate datasource. It generates either:

• datasource.ts, or;
• datasource.json with DataSource class that reads datasource.json

Sample for the first approach:

• https://github.com/strongloop/loopback-next/blob/master/packages/repository/examples/juggler-bridge/note-with-repo-class.ts#L22-L25

Sample for the latter approach:

• datasource.json: https://github.com/strongloop/loopback-next/blob/master/packages/example-todo/config/datasources.json
• datasource.ts: https://github.com/strongloop/loopback-next/blob/master/packages/example-todo/src/datasources/db.datasource.ts

User Experience

$lb datasource
? Enter the datasource name: testds
? Select the connector for testds: (selection of connectors) PostgreSQL (supported by StrongLoop)
? Connection String url to override other settings (eg: postgres://username:password@localhost/database): postgres://username:password@localhost/database
? host: localhost
? port: 5432
? user: username
? password: ****
? database: testdb
? Install loopback-connector-postgresql@^2.4 No

As a result, src/datasources/testds.datasource.ts will be generated.

Outstanding questions

• What should we be generating? just the datasource.ts? or similar to what we have in example-todo?
• For the list of connectors for selection, do we want to have the same list as in LB3?

---

Acceptance Criteria

• Create lb4 datasource -- works like LB3 CLI with the following changes:
  • Each datasource config is stored in it's own .json file
  • An accompanying .ts file is created which injects the datasource config
• Tests as usual
• Not related but relevant: @loopback/boot will bind JSON config files to Context
  • Must provide a way to override from env ...

[dhmlau]

Things to consider:

1. CLI to generate the datasource
2. bootstrap any LB3 datasources.json and model definition.

[bajtos]

What should we be generating? just the datasource.ts? or similar to what we have in example-todo?

IMO, we should also register the newly created datasource with the application, see e.g. https://github.com/strongloop/loopback-next/blob/c92603157ca7f08cf3748ca6bbd1e68da87d821e/examples/todo/src/application.ts#L48-L56

Simplified version that's probably good enough for DP3:

import {db} from './db.datasource.ts';
// ...
app.dataSource(db);

Ideally, this registration should provided by @loopback/boot, but I think we are not there yet. I guess we should create a new story to allow @loopback/boot to discover and register datasources and implement that story first, before working on this one.

For the list of connectors for selection, do we want to have the same list as in LB3?

Yes, let's start with the same list.

[jannyHou]

Ideally, this registration should provided by @loopback/boot

@bajtos see #1261, I think @virkt25 is already doing it :)

[dhmlau]

related to #441

[dhmlau]

@virkt25 , assigning this to you because I think you're already doing most of it as part of the spike #441

[dhmlau]

@virkt25 , if it's good for estimate, please move to the needs estimate column. thanks.