Migrations in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this migration script execution?

Consider a NestJS migration that adds a new column to an existing table. What will be the state of the database after running this migration?

NestJS

import { MigrationInterface, QueryRunner } from 'typeorm';

export class AddAgeColumn1680000000000 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`ALTER TABLE users ADD COLUMN age int`);
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`ALTER TABLE users DROP COLUMN age`);
  }
}

A The 'users' table will have a new column named 'age' of type integer.

B The migration will fail because 'ALTER TABLE' is not supported in migrations.

C The 'users' table will be deleted and recreated with the 'age' column.

D No changes will be made to the database because the migration script is empty.

💡 Hint

Think about what the up method does in a migration.

The up method runs the SQL to add the new column. This changes the database schema by adding the 'age' column to the 'users' table.

📝 Syntax

intermediate

Which option contains a syntax error in a NestJS migration?

Identify the migration code snippet that will cause a syntax error when executed.

A await queryRunner.query(`ALTER TABLE users ADD age int`);

B await queryRunner.query('ALTER TABLE users DROP COLUMN age');

C await queryRunner.query(`ALTER TABLE users ADD COLUMN created_at timestamp`);

D await queryRunner.query(`ALTER TABLE users ADD COLUMN email varchar(255)`);

💡 Hint

Check the SQL syntax for adding a column.

Option A is missing the keyword COLUMN after ADD, which is required in SQL to add a new column.

🔧 Debug

advanced

Why does this migration fail to rollback properly?

Given the migration below, why does running the down method cause an error?

NestJS

import { MigrationInterface, QueryRunner } from 'typeorm';

export class RenameUsernameToUser1680000000001 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`ALTER TABLE users RENAME COLUMN username TO user`);
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`ALTER TABLE users RENAME COLUMN user TO username`);
  }
}

A The down method should drop the 'user' column instead of renaming.

B The migration fails because 'RENAME COLUMN' is not supported in SQL.

C The down method tries to rename 'username' to 'user' again, but 'username' no longer exists after up migration.

D The migration fails because the table 'users' does not exist.

💡 Hint

Think about the state of the database after the up method runs.

After the up method, the column 'username' is renamed to 'user'. The down method tries to rename 'username' again, but it no longer exists, causing an error.

❓ state_output

advanced

What is the state of the database after running this migration?

This migration creates a new table. What will the database contain after running up?

NestJS

import { MigrationInterface, QueryRunner } from 'typeorm';

export class CreateProductsTable1680000000002 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`CREATE TABLE products (id int PRIMARY KEY, name varchar(100), price decimal)`);
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`DROP TABLE products`);
  }
}

A The migration will create a table named 'product' (singular) instead of 'products'.

B The migration will fail because 'decimal' is not a valid column type.

C The 'products' table will be created but without any columns.

D The database will have a new table named 'products' with columns 'id', 'name', and 'price'.

💡 Hint

Check the SQL syntax for creating tables and column definitions.

The up method creates a table named 'products' with the specified columns and types. The SQL syntax is correct.

🧠 Conceptual

expert

Which option best describes the purpose of the 'down' method in NestJS migrations?

Why is the down method important in a migration class?

A It runs automatically before the <code>up</code> method to prepare the database.

B It defines how to revert the changes made by the <code>up</code> method, allowing rollback of the migration.

C It is used to seed initial data into the database after migration.

D It validates the migration syntax before execution.

💡 Hint

Think about what happens if you want to undo a migration.

The down method provides instructions to undo the changes made by up. This is essential for safely rolling back migrations if needed.