fix(sequelize): init Sequelize instance in constructor

This resolves initialization errors in Loopback 4 observers
attempting to use repositories extending the Sequelize CRUD repository.

Signed-off-by: KalleV <kvirtaneva@gmail.com>

Author: KalleV

extensions/sequelize/src/__tests__/fixtures/observers/index.ts

@@ -0,0 +1 @@
+export * from './test.observer';

extensions/sequelize/src/__tests__/fixtures/observers/test.observer.ts

@@ -0,0 +1,26 @@
+import {lifeCycleObserver, type LifeCycleObserver} from '@loopback/core';
+import {repository} from '@loopback/repository';
+import {UserRepository} from '../repositories';
+
+/**
+ * Test observer for validating that the Sequelize repositories are available in Loopback Observers during server startup.
+ */
+@lifeCycleObserver('test')
+export class TestObserver implements LifeCycleObserver {
+  constructor(
+    @repository(UserRepository)
+    private userRepository: UserRepository,
+  ) {}
+
+  async start(): Promise<void> {
+    try {
+      await this.userRepository.find();
+    } catch (error) {
+      // For the repository tests, the database schema is not created until after the server is initialized:
+      // extensions/sequelize/src/__tests__/integration/repository.integration.ts
+      if (!error.message.includes('no such table: User')) {
+        throw error;
+      }
+    }
+  }
+}

extensions/sequelize/src/__tests__/integration/repository.integration.ts

@@ -1114,6 +1114,7 @@ describe('Sequelize CRUD Repository (integration)', () => {
         'task.controller',
         'scoped-task.controller',
       ],
+      observers: ['index', 'test.observer'],
     };
 
     const copyFilePromises: Array<Promise<unknown>> = [];

extensions/sequelize/src/sequelize/sequelize.datasource.base.ts

@@ -45,12 +45,7 @@ export class SequelizeDataSource implements LifeCycleObserver {
         } is not supported.`,
       );
     }
-  }
 
-  sequelize?: Sequelize;
-  sequelizeConfig: SequelizeOptions;
-  async init(): Promise<void> {
-    const {config} = this;
     const {
       connector,
       file,
@@ -86,7 +81,11 @@ export class SequelizeDataSource implements LifeCycleObserver {
     } else {
       this.sequelize = new Sequelize(this.sequelizeConfig);
     }
+  }
 
+  sequelize: Sequelize;
+  sequelizeConfig: SequelizeOptions;
+  async init(): Promise<void> {
     await this.sequelize.authenticate();
     debug('Connection has been established successfully.');
   }