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

Concept Flow - Migrations for schema changes

Write migration file

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Run migration command

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Migration tool reads file

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Apply schema changes to DB

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Update migration history

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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.