docs: add automigrate/autoupdate

docs/site/Database-migrations.md

@@ -0,0 +1,142 @@
+---
+lang: en
+title: 'Database Migrations'
+keywords: LoopBack 4.0
+sidebar: lb4_sidebar
+permalink: /doc/en/lb4/Database-migrations.html
+---
+
+## Overview
+
+In LoopBack, auto-migration helps the user create relational database schemas
+based on definitions of their models. Auto-migration can facilitate the
+synchronization of the backing database and models so that they match, such as
+in cases where the database needs to be changed in order to match the models.
+LoopBack offers two ways to do this:
+
+- **Auto-migrate**: Drop schema objects if they already exist and re-create them
+  based on model definitions. Existing data will be lost.
+
+- **Auto-update**: Change database schema objects if there is a difference
+  between the objects and model definitions. Existing data will be kept.
+
+## Implementation Example
+
+Below is an example of how to implement
+[automigrate()](http://apidocs.loopback.io/loopback-datasource-juggler/#datasource-prototype-automigrate)
+and
+[autoupdate()](http://apidocs.loopback.io/loopback-datasource-juggler/#datasource-prototype-autoupdate),
+shown with the
+[TodoList](https://loopback.io/doc/en/lb4/todo-list-tutorial.html) example.
+
+Create a new file **src/migrate.ts** and add the following import statement:
+
+```ts
+import {DataSource, Repository} from '@loopback/repository';
+```
+
+Import your application and your repositories:
+
+```ts
+import {TodoListApplication} from './index';
+import {TodoRepository, TodoListRepository} from './repositories';
+```
+
+Create a function called _dsMigrate()_:
+
+```ts
+export async function dsMigrate(app: TodoListApplication) {}
+```
+
+In the _dsMigrate()_ function, get your datasource and initiate your
+repositories by retrieving them, so that the models are attached to the
+corresponding datasource:
+
+```ts
+const ds = await app.get<DataSource>('datasources.db');
+const todoRepo = await app.getRepository(TodoRepository);
+const todoListRepo = await app.getRepository(TodoListRepository);
+```
+
+Then, in the same function, call _automigrate()_:
+
+```ts
+await ds.automigrate();
+```
+
+This call to automigrate will migrate all the models attached to the datasource
+db. However if you want to only migrate some of your models, add the names of
+the classes in the first parameter:
+
+```ts
+// Migrate a single model
+ds.automigrate('Todo');
+```
+
+```ts
+// Migrate multiple models
+ds.automigrate(['Todo', 'TodoList']);
+```
+
+The implementation for _autoupdate()_ is similar. Create a new function
+_dsUpdate()_:
+
+```ts
+export async function dsUpdate(app: TodoListApplication) {
+  const ds = await app.get<DataSource>('datasources.db');
+  const todoRepo = await app.getRepository(TodoRepository);
+  const todoListRepo = await app.getRepository(TodoListRepository);
+
+  await ds.autoupdate();
+}
+```
+
+The completed **src/migrate.ts** should look similar to this:
+
+```ts
+import {DataSource, Repository} from '@loopback/repository';
+import {TodoListApplication} from './index';
+import {TodoRepository, TodoListRepository} from './repositories';
+
+export async function dsMigrate(app: TodoListApplication) {
+  const ds = await app.get<DataSource>('datasources.db');
+  const todoRepo = await app.getRepository(TodoRepository);
+  const todoListRepo = await app.getRepository(TodoListRepository);
+
+  await ds.automigrate();
+}
+
+export async function dsUpdate(app: TodoListApplication) {
+  const ds = await app.get<DataSource>('datasources.db');
+  const todoRepo = await app.getRepository(TodoRepository);
+  const todoListRepo = await app.getRepository(TodoListRepository);
+
+  await ds.autoupdate();
+}
+```
+
+Finally, in **src/index.ts**, import and call the _dsMigrate()_ or _dsUpdate()_
+function:
+
+```ts
+import {TodoListApplication} from './application';
+import {ApplicationConfig} from '@loopback/core';
+
+// Import the functions from src/migrate.ts
+import {dsMigrate, dsUpdate} from './migrate';
+
+export {TodoListApplication};
+
+export async function main(options: ApplicationConfig = {}) {
+  const app = new TodoListApplication(options);
+  await app.boot();
+  await app.start();
+
+  const url = app.restServer.url;
+  console.log(`Server is running at ${url}`);
+
+  // The call to dsMigrate(), or replace with dsUpdate()
+  await dsMigrate(app);
+  return app;
+}
+```

docs/site/sidebars/lb4_sidebar.yml

@@ -203,6 +203,10 @@ children:
       url: Deploying-to-IBM-Cloud.html
       output: 'web, pdf'
 
+    - title: 'Database Migrations'
+      url: Database-migrations.html
+      output: 'web, pdf'
+
 - title: 'Booting an Application'
   url: Booting-an-Application.html
   output: 'web, pdf'