How to delete the columns from the typeorm entities

When working with TypeORM for database operations, you may need to remove columns from entities. This is often due to changes in business requirements where certain data fields are no longer needed to be stored. Removing a column requires careful handling to avoid data loss and application crashes. The following are the steps to remove columns from TypeORM entities, along with considerations to keep in mind:

Step 1: Update the Entity Model

First, update the entity model to remove unnecessary columns. For example, assume we have an entity named User that includes a column named age, which we now need to remove:

typescript
import { Entity, PrimaryGeneratedColumn, Column } from "typeorm";

@Entity()
export class User {
    @PrimaryGeneratedColumn()
    id: number;

    @Column()
    name: string;

    // @Column()
    // age: number;  // Remove or comment out this line
}

Step 2: Migrate the Database

After removing columns from the model, update the database to reflect these changes. In TypeORM, use migrations to manage database structure changes. Run the following command to generate a new migration file:

bash
typeorm migration:generate -n RemoveAgeColumn

This will generate a new file in your migration folder, which you need to edit to specify how to update the database. For example:

typescript
import {MigrationInterface, QueryRunner} from "typeorm";

export class RemoveAgeColumn1614000000000 implements MigrationInterface {
    public async up(queryRunner: QueryRunner): Promise<void> {
        await queryRunner.query(`ALTER TABLE "user" DROP COLUMN "age"`);
    }

    public async down(queryRunner: QueryRunner): Promise<void> {
        await queryRunner.query(`ALTER TABLE "user" ADD "age" int`);
    }
}

The up method describes how to apply the migration, while the down method describes how to revert the migration if needed.

Step 3: Execute the Migration

Finally, execute the migration to update the database:

bash
typeorm migration:run

Considerations

• Data Backup: Back up relevant data before removing the column in case you need to restore it.
• Code Dependencies: Ensure the column to be removed is not referenced elsewhere in the application, as this could lead to runtime errors.
• Testing: Thoroughly test these changes in a development or testing environment before applying them in production.

By following these steps, you can safely remove columns from TypeORM entities while ensuring application stability and data integrity.