Fix: How to get updated column using typeorm EventSubscriber afterTransactionCommit?

 In TypeORM, you can use an `EventSubscriber` to listen for various database events, including events that occur after a transaction commit. To get the updated columns after a transaction commit, you can utilize the `afterTransactionCommit` event. Here's how you can achieve this:


1. **Create an EventSubscriber**:


   First, create an `EventSubscriber` class that listens for the `afterTransactionCommit` event.


   ```typescript

   import { EventSubscriber, EntitySubscriberInterface, EventSubscriberInterface, InsertEvent, UpdateEvent, RemoveEvent, Entity, EntityManager } from 'typeorm';


   @EventSubscriber()

   export class YourEventSubscriber implements EntitySubscriberInterface<Entity> {

       constructor(

           private readonly entityManager: EntityManager,

       ) {

           this.entityManager = entityManager;

           this.entityManager.registerSubscriber(this);

       }


       listenTo() {

           return Entity; // Replace 'Entity' with the entity class you want to monitor.

       }


       afterTransactionCommit(event: InsertEvent<any> | UpdateEvent<any> | RemoveEvent<any>) {

           // Check the event type (InsertEvent, UpdateEvent, or RemoveEvent)

           if (event instanceof UpdateEvent) {

               const updatedColumns = event.updatedColumns.map(column => column.propertyName);

               console.log('Updated Columns:', updatedColumns);

           }

       }

   }

   ```


   In this example, we listen for the `afterTransactionCommit` event and handle it for the specified entity class (replace `'Entity'` with the entity you want to monitor). We specifically look for `UpdateEvent` to retrieve information about the updated columns.


2. **Register the Event Subscriber**:


   You need to register the `EventSubscriber` in your application to ensure it's active. You can do this when configuring your TypeORM connection.


   ```typescript

   import { createConnection } from 'typeorm';


   createConnection({

       type: 'your_database_type',

       host: 'your_database_host',

       port: 5432, // Your database port

       username: 'your_database_username',

       password: 'your_database_password',

       database: 'your_database_name',

       subscribers: [YourEventSubscriber],

       // ...

   }).then(async (connection) => {

       // Your application logic

   }).catch(error => console.log(error));

   ```


3. **Usage and Testing**:


   With your `EventSubscriber` in place, you can now observe and log the updated columns of the specified entity every time a transaction is committed.


Keep in mind that the example provided above listens for the `UpdateEvent` within the `afterTransactionCommit` event. You can extend this logic to handle other events (inserts and deletes) and more complex use cases based on your application's needs. Additionally, ensure that you replace `'Entity'` with the actual entity class you want to monitor.

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