Migrations for schema changes in Express - Step-by-Step Execution

Concept Flow - Migrations for schema changes

Write migration file

↓

Run migration command

↓

Migration tool reads file

↓

Apply schema changes to DB

↓

Update migration history

↓

App uses updated schema

This flow shows how you write a migration, run it, and the database schema updates step-by-step.

Execution Sample

Express

npx sequelize-cli migration:generate --name add-users-table
// Edit migration file to add table
npx sequelize-cli db:migrate

This example generates a migration file, edits it to add a users table, then runs the migration to update the database.

Execution Table

Step	Action	Tool Output	Database State	Migration History
1	Generate migration file	Created file: add-users-table.js	No change	No new entry
2	Edit migration file	File updated with createTable code	No change	No new entry
3	Run migration command	Executing migration scripts...	Users table created	Migration 'add-users-table' recorded
4	Verify migration	Migration successful	Users table exists	Migration recorded
5	Run migration again	No pending migrations	No change	No new entry

💡 No pending migrations left to apply, database schema is up to date

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Migration Files	[]	[add-users-table.js]	[add-users-table.js]	[add-users-table.js]	[add-users-table.js]	[add-users-table.js]
Database Schema	Initial schema	Initial schema	Initial schema	Schema + users table	Schema + users table	Schema + users table
Migration History	[]	[]	[]	['add-users-table']	['add-users-table']	['add-users-table']

Key Moments - 2 Insights

Why does running the migration command twice not change the database the second time?

Because the migration history shows the migration was already applied (see step 5 in execution_table), so the tool skips it to avoid duplicate changes.

What happens if you forget to edit the generated migration file before running it?

The migration runs but makes no schema changes because the file has no instructions (see step 2 and 3 in execution_table). The database stays the same.

Visual Quiz - 3 Questions

Look at the execution table, what is the database state after step 3?

A No change

B Users table created

C Migration history empty

D Migration failed

💡 Hint

Check the 'Database State' column at step 3 in the execution_table.

At which step does the migration history get updated?

A Step 3

B Step 5

C Step 1

D Step 2

💡 Hint

Look at the 'Migration History' column in execution_table rows.

If you run the migration command again after step 4, what will the tool output say?

A Executing migration scripts...

B Error: migration file missing

C No pending migrations

D Migration successful

💡 Hint

See step 5 in execution_table for repeated migration run output.

Concept Snapshot

Migrations help update database schema safely.
Write migration files describing changes.
Run migration commands to apply changes.
Migration history tracks applied changes.
Running migrations twice skips already applied ones.

Full Transcript

Migrations for schema changes in Express apps involve creating migration files that describe how to change the database structure. You generate a migration file, edit it to add or modify tables or columns, then run a migration command. The migration tool reads the file and applies the changes to the database. It also records the migration in a history log to avoid repeating the same changes. Running migrations again after all are applied results in no action. This process helps keep the database schema in sync with your app code safely and predictably.

Practice

1. What is the main purpose of migrations in an Express app using a database?

easy

A. To create API routes for the app

B. To handle user authentication automatically

C. To manage and apply database schema changes step-by-step

D. To serve static files like images and CSS

Solution

1. Step 1: Understand migrations role

   Migrations help track and apply changes to the database schema in a controlled way.

2. Step 2: Compare other options

   Options A, B, and C relate to other app features, not schema changes.

3. Final Answer:

   To manage and apply database schema changes step-by-step -> Option C

4. Quick Check:

   Migrations = Manage schema changes [OK]

Hint: Migrations = step-by-step database structure updates [OK]

Common Mistakes:

• Confusing migrations with routing or middleware
• Thinking migrations handle user login
• Assuming migrations serve static files

2. Which of the following is the correct command to create a new migration file using a common migration tool like Sequelize CLI?

easy

A. sequelize new migration add-users-table

B. sequelize migrate new add-users-table

C. sequelize create migration add-users-table

D. sequelize migration:create --name add-users-table

Solution

1. Step 1: Recall Sequelize CLI syntax

   The correct command to create a migration is sequelize migration:create --name <name>.

2. Step 2: Check other options

   Options B, C, and D use incorrect command structures not recognized by Sequelize CLI.

3. Final Answer:

   sequelize migration:create --name add-users-table -> Option D

4. Quick Check:

   Correct CLI command = sequelize migration:create --name add-users-table [OK]

Hint: Use 'migration:create --name' to create migration files [OK]

Common Mistakes:

• Mixing up command order or keywords
• Omitting '--name' flag
• Using 'new' instead of 'create'

3. Given this migration snippet using Sequelize:

module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn('Users', 'age', { type: Sequelize.INTEGER });
  },
  down: async (queryInterface) => {
    await queryInterface.removeColumn('Users', 'age');
  }
};

What will happen when this migration is run?

medium

A. Creates a new 'Users' table with an 'age' column

B. Adds an 'age' column of type INTEGER to the 'Users' table

C. Removes the 'age' column from the 'Users' table

D. Throws an error because 'down' is missing Sequelize parameter

Solution

1. Step 1: Analyze the 'up' function

   The 'up' function adds a new column 'age' of type INTEGER to the 'Users' table.

2. Step 2: Understand the 'down' function

   The 'down' function reverses this by removing the 'age' column, but this runs only on rollback.

3. Final Answer:

   Adds an 'age' column of type INTEGER to the 'Users' table -> Option B

4. Quick Check:

   'up' adds column, 'down' removes column [OK]

Hint: 'up' applies changes, 'down' reverts them [OK]

Common Mistakes:

• Confusing 'up' and 'down' functions
• Assuming 'down' runs on migration
• Thinking it creates a new table

4. You wrote this migration to rename a column but it throws an error:

module.exports = {
  up: async (queryInterface) => {
    await queryInterface.renameColumn('Users', 'username', 'user_name');
  },
  down: async (queryInterface) => {
    await queryInterface.renameColumn('Users', 'user_name', 'username');
  }
};

What is the most likely cause of the error?

medium

A. The table name 'Users' is incorrect or does not exist

B. Missing Sequelize parameter in the functions

C. renameColumn requires the column type as a third argument

D. The migration file name is invalid

Solution

1. Step 1: Check renameColumn usage

   renameColumn needs correct table and column names; no type argument is needed.

2. Step 2: Consider error causes

   If the table 'Users' does not exist or is misnamed, renameColumn will fail.

3. Final Answer:

   The table name 'Users' is incorrect or does not exist -> Option A

4. Quick Check:

   Table name must exist for renameColumn [OK]

Hint: Check table and column names exist before renaming [OK]

Common Mistakes:

• Assuming renameColumn needs column type
• Ignoring table name case sensitivity
• Forgetting to verify table existence

5. You want to add a new column 'email' to the 'Users' table but only if it doesn't already exist, to avoid errors during migration. Which approach best handles this in an Express app migration?

hard

A. Use a raw SQL query inside the migration to check column existence before adding

B. Directly add the column without checks; rely on migration rollback if error occurs

C. Modify the model only and skip migration since model syncs automatically

D. Delete the 'Users' table first, then add the column

Solution

1. Step 1: Understand migration safety

   Adding a column without checking can cause errors if it exists already.

2. Step 2: Use raw SQL for conditional check

   Running a raw SQL query to check if the column exists before adding prevents errors.

3. Step 3: Evaluate other options

   Directly add the column without checks; rely on migration rollback if error occurs risks errors, C ignores migration purpose, D is destructive and unsafe.

4. Final Answer:

   Use a raw SQL query inside the migration to check column existence before adding -> Option A

5. Quick Check:

   Check column first with SQL to avoid errors [OK]

Hint: Check column existence with SQL before adding [OK]

Common Mistakes:

• Skipping existence check causing migration failure
• Relying only on model sync without migration
• Dropping tables unnecessarily