Database migrations in NextJS - Step-by-Step Execution

Concept Flow - Database migrations

Write migration file

↓

Run migration command

↓

Migration tool reads file

↓

Apply schema changes to DB

↓

Update migration history

↓

App uses updated DB schema

This flow shows how writing and running a migration file updates the database schema step-by-step.

Execution Sample

NextJS

import { migrate } from '@nextjs/db-migrate';

await migrate('2024_06_add_users_table');

This code runs a migration named '2024_06_add_users_table' to update the database schema.

Execution Table

Step	Action	Input	Result	DB State Change
1	Write migration file	'2024_06_add_users_table' with SQL to create users table	Migration file created	No change yet
2	Run migration command	migrate('2024_06_add_users_table')	Migration tool starts	No change yet
3	Read migration file	Read SQL commands	SQL commands loaded	No change yet
4	Apply schema changes	Execute SQL: CREATE TABLE users (...)	Users table created	Users table added
5	Update migration history	Record migration applied	Migration recorded	Migration history updated
6	Finish migration	Return success	Migration complete	DB schema updated
7	Exit	No more migrations	Process ends	Final DB schema with users table

💡 All migration steps completed successfully; database schema is now updated.

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
migrationFile	undefined	'2024_06_add_users_table' content	'2024_06_add_users_table' content	'2024_06_add_users_table' content
dbSchema	Initial schema	Users table added	Users table added	Users table added
migrationHistory	Empty	Empty	Contains '2024_06_add_users_table'	Contains '2024_06_add_users_table'

Key Moments - 2 Insights

Why doesn't the database schema change when I just write the migration file?

Writing the migration file only prepares the instructions. The schema changes only happen when you run the migration command, as shown in steps 2 to 4 in the execution table.

What happens if I run the migration command twice?

The migration history (step 5) prevents reapplying the same migration. The tool checks history and skips already applied migrations to avoid errors.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the users table actually created in the database?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Check the 'DB State Change' column for when the users table is added.

According to the variable tracker, what is the state of migrationHistory after step 5?

A Empty

B Contains '2024_06_add_users_table'

C Undefined

D Reset to initial

💡 Hint

Look at the migrationHistory row under 'After Step 5' in variable_tracker.

If the migration file is missing, what step would fail in the execution table?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Step 3 reads the migration file; if missing, it cannot load SQL commands.

Concept Snapshot

Database migrations:
- Write migration files with schema changes
- Run migration command to apply changes
- Migration tool reads files and updates DB
- Migration history tracks applied changes
- Prevents duplicate or missing updates

Full Transcript

Database migrations in Next.js involve writing migration files that describe changes to the database schema. When you run the migration command, the tool reads these files and applies the changes to the database. It also records which migrations have been applied to avoid repeating them. This process updates the database schema safely and in order.

Practice

1.

What is the main purpose of database migrations in a Next.js project?

easy

A. To deploy the Next.js app to the server

B. To safely update the database structure as the app evolves

C. To create user interfaces automatically

D. To write CSS styles for the app

Solution

1. Step 1: Understand database migrations

   Database migrations are used to change the database schema safely without losing data.

2. Step 2: Relate to Next.js usage

   In Next.js, migrations help keep the database structure in sync with app changes.

3. Final Answer:

   To safely update the database structure as the app evolves -> Option B

4. Quick Check:

   Database migrations = safe schema updates [OK]

Hint: Migrations change database structure safely [OK]

Common Mistakes:

• Confusing migrations with UI or styling tasks
• Thinking migrations deploy the app
• Assuming migrations create app features

2.

Which command is used to create and apply a new migration in a Next.js project using Prisma?

?

easy

A. next dev

B. npm run build

C. npx prisma migrate dev

D. npx prisma generate

Solution

1. Step 1: Identify Prisma migration command

   The command npx prisma migrate dev creates and applies migrations during development.

2. Step 2: Differentiate from other commands

   npm run build builds the app, next dev runs dev server, and npx prisma generate regenerates Prisma client but does not migrate.

3. Final Answer:

   npx prisma migrate dev -> Option C

4. Quick Check:

   Migration command = npx prisma migrate dev [OK]

Hint: Use 'npx prisma migrate dev' to create and apply migrations [OK]

Common Mistakes:

• Using 'npm run build' to migrate database
• Confusing 'npx prisma generate' with migration
• Running 'next dev' expecting migration

3.

Given this Prisma schema change and migration command, what will happen?

model User {
  id    Int    @id @default(autoincrement())
  email String @unique
  name  String?
}

// After adding a new field:
model User {
  id       Int    @id @default(autoincrement())
  email    String @unique
  name     String?
  isActive Boolean @default(true)
}

Command: npx prisma migrate dev --name add-isActive

medium

A. A new migration file is created and applied adding the isActive column with default true

B. The migration fails because isActive is missing in existing rows

C. The database schema remains unchanged

D. The app crashes due to schema mismatch

Solution

1. Step 1: Understand schema change

   A new Boolean field isActive with default true is added to the User model.

2. Step 2: Effect of migration command

   The command creates a migration file that adds the new column with default value, so existing rows get true automatically.

3. Final Answer:

   A new migration file is created and applied adding the isActive column with default true -> Option A

4. Quick Check:

   Adding field with default = migration applies safely [OK]

Hint: Adding field with default creates migration safely [OK]

Common Mistakes:

• Thinking migration fails due to missing data for new field
• Assuming schema does not change without manual SQL
• Believing app crashes immediately after migration

4.

What is wrong with this migration command usage in a Next.js project?

npx prisma migrate dev --name

medium

A. The command is correct and will run successfully

B. The command should be run with npm, not npx

C. The command should be prisma migrate deploy instead

D. The migration name is missing after --name flag

Solution

1. Step 1: Check command syntax

   The --name flag requires a migration name string after it.

2. Step 2: Identify missing argument

   Here, no name is provided, so the command will error out.

3. Final Answer:

   The migration name is missing after --name flag -> Option D

4. Quick Check:

   Migration name required after --name [OK]

Hint: Always provide a name after --name flag [OK]

Common Mistakes:

• Omitting migration name after --name
• Confusing migrate dev with migrate deploy
• Using npm instead of npx unnecessarily

5.

You want to add a new required field age Int to your User model in Prisma, but your database already has users without this field. What is the best way to handle this migration?

hard

A. Add age Int? as optional first, migrate, then backfill data, then make it required

B. Add age Int directly and run migration; it will succeed automatically

C. Delete all existing users before migration to avoid errors

D. Skip migration and add the field only in the app code

Solution

1. Step 1: Understand required field constraints

   Adding a required field age Int directly will fail because existing rows lack this data.

2. Step 2: Use a safe migration approach

   First add the field as optional (age Int?), migrate, then update existing data, and finally change it to required.

3. Final Answer:

   Add age Int? as optional first, migrate, then backfill data, then make it required -> Option A

4. Quick Check:

   Make new required fields optional first [OK]

Hint: Add new required fields as optional first [OK]

Common Mistakes:

• Adding required field directly causing migration failure
• Deleting data unnecessarily
• Skipping migration and causing runtime errors