Database Migration

Master database schema and data migrations across ORMs (Sequelize, TypeORM, Prisma), including rollback strategies and zero-downtime deployments.

When to Use This Skill

• Migrating between different ORMs
• Performing schema transformations
• Moving data between databases
• Implementing rollback procedures
• Zero-downtime deployments
• Database version upgrades
• Data model refactoring

ORM Migrations

Sequelize Migrations

// migrations/20231201-create-users.js
module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.createTable("users", {
      id: {
        type: Sequelize.INTEGER,
        primaryKey: true,
        autoIncrement: true,
      },
      email: {
        type: Sequelize.STRING,
        unique: true,
        allowNull: false,
      },
      createdAt: Sequelize.DATE,
      updatedAt: Sequelize.DATE,
    });
  },

  down: async (queryInterface, Sequelize) => {
    await queryInterface.dropTable("users");
  },
};

// Run: npx sequelize-cli db:migrate
// Rollback: npx sequelize-cli db:migrate:undo

TypeORM Migrations

// migrations/1701234567-CreateUsers.ts
import { MigrationInterface, QueryRunner, Table } from "typeorm";

export class CreateUsers1701234567 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.createTable(
      new Table({
        name: "users",
        columns: [
          {
            name: "id",
            type: "int",
            isPrimary: true,
            isGenerated: true,
            generationStrategy: "increment",
          },
          {
            name: "email",
            type: "varchar",
            isUnique: true,
          },
          {
            name: "created_at",
            type: "timestamp",
            default: "CURRENT_TIMESTAMP",
          },
        ],
      }),
    );
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.dropTable("users");
  }
}

// Run: npm run typeorm migration:run
// Rollback: npm run typeorm migration:revert

Prisma Migrations

// schema.prisma
model User {
  id        Int      @id @default(autoincrement())
  email     String   @unique
  createdAt DateTime @default(now())
}

// Generate migration: npx prisma migrate dev --name create_users
// Apply: npx prisma migrate deploy

Schema Transformations

Adding Columns with Defaults

// Safe migration: add column with default
module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn("users", "status", {
      type: Sequelize.STRING,
      defaultValue: "active",
      allowNull: false,
    });
  },

  down: async (queryInterface) => {
    await queryInterface.removeColumn("users", "status");
  },
};

Renaming Columns (Zero Downtime)

// Step 1: Add new column
module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn("users", "full_name", {
      type: Sequelize.STRING,
    });

    // Copy data from old column
    await queryInterface.sequelize.query("UPDATE users SET full_name = name");
  },

  down: async (queryInterface) => {
    await queryInterface.removeColumn("users", "full_name");
  },
};

// Step 2: Update application to use new column

// Step 3: Remove old column
module.exports = {
  up: async (queryInterface) => {
    await queryInterface.removeColumn("users", "name");
  },

  down: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn("users", "name", {
      type: Sequelize.STRING,
    });
  },
};

Changing Column Types

module.exports = {
  up: async (queryInterface, Sequelize) => {
    // For large tables, use multi-step approach

    // 1. Add new column
    await queryInterface.addColumn("users", "age_new", {
      type: Sequelize.INTEGER,
    });

    // 2. Copy and transform data
    await queryInterface.sequelize.query(`
      UPDATE users
      SET age_new = CAST(age AS INTEGER)
      WHERE age IS NOT NULL
    `);

    // 3. Drop old column
    await queryInterface.removeColumn("users", "age");

    // 4. Rename new column
    await queryInterface.renameColumn("users", "age_new", "age");
  },

  down: async (queryInterface, Sequelize) => {
    await queryInterface.changeColumn("users", "age", {
      type: Sequelize.STRING,
    });
  },
};

Data Transformations

Complex Data Migration

module.exports = {
  up: async (queryInterface, Sequelize) => {
    // Get all records
    const [users] = await queryInterface.sequelize.query(
      "SELECT id, address_string FROM users",
    );

    // Transform each record
    for (const user of users) {
      const addressParts = user.address_string.split(",");

      await queryInterface.sequelize.query(
        `UPDATE users
         SET street = :street,
             city = :city,
             state = :state
         WHERE id = :id`,
        {
          replacements: {
            id: user.id,
            street: addressParts[0]?.trim(),
            city: addressParts[1]?.trim(),
            state: addressParts[2]?.trim(),
          },
        },
      );
    }

    // Drop old column
    await queryInterface.removeColumn("users", "address_string");
  },

  down: async (queryInterface, Sequelize) => {
    // Reconstruct original column
    await queryInterface.addColumn("users", "address_string", {
      type: Sequelize.STRING,
    });

    await queryInterface.sequelize.query(`
      UPDATE users
      SET address_string = CONCAT(street, ', ', city, ', ', state)
    `);

    await queryInterface.removeColumn("users", "street");
    await queryInterface.removeColumn("users", "city");
    await queryInterface.removeColumn("users", "state");
  },
};

Rollback Strategies

Transaction-Based Migrations

module.exports = {
  up: async (queryInterface, Sequelize) => {
    const transaction = await queryInterface.sequelize.transaction();

    try {
      await queryInterface.addColumn(
        "users",
        "verified",
        { type: Sequelize.BOOLEAN, defaultValue: false },
        { transaction },
      );

      await queryInterface.sequelize.query(
        "UPDATE users SET verified = true WHERE email_verified_at IS NOT NULL",
        { transaction },
      );

      await transaction.commit();
    } catch (error) {
      await transaction.rollback();
      throw error;
    }
  },

  down: async (queryInterface) => {
    await queryInterface.removeColumn("users", "verified");
  },
};

Checkpoint-Based Rollback

module.exports = {
  up: async (queryInterface, Sequelize) => {
    // Create backup table
    await queryInterface.sequelize.query(
      "CREATE TABLE users_backup AS SELECT * FROM users",
    );

    try {
      // Perform migration
      await queryInterface.addColumn("users", "new_field", {
        type: Sequelize.STRING,
      });

      // Verify migration
      const [result] = await queryInterface.sequelize.query(
        "SELECT COUNT(*) as count FROM users WHERE new_field IS NULL",
      );

      if (result[0].count > 0) {
        throw new Error("Migration verification failed");
      }

      // Drop backup
      await queryInterface.dropTable("users_backup");
    } catch (error) {
      // Restore from backup
      await queryInterface.sequelize.query("DROP TABLE users");
      await queryInterface.sequelize.query(
        "CREATE TABLE users AS SELECT * FROM users_backup",
      );
      await queryInterface.dropTable("users_backup");
      throw error;
    }
  },
};

Zero-Downtime Migrations

Blue-Green Deployment Strategy

// Phase 1: Make changes backward compatible
module.exports = {
  up: async (queryInterface, Sequelize) => {
    // Add new column (both old and new code can work)
    await queryInterface.addColumn("users", "email_new", {
      type: Sequelize.STRING,
    });
  },
};

// Phase 2: Deploy code that writes to both columns

// Phase 3: Backfill data
module.exports = {
  up: async (queryInterface) => {
    await queryInterface.sequelize.query(`
      UPDATE users
      SET email_new = email
      WHERE email_new IS NULL
    `);
  },
};

// Phase 4: Deploy code that reads from new column

// Phase 5: Remove old column
module.exports = {
  up: async (queryInterface) => {
    await queryInterface.removeColumn("users", "email");
  },
};

Cross-Database Migrations

PostgreSQL to MySQL

// Handle differences
module.exports = {
  up: async (queryInterface, Sequelize) => {
    const dialectName = queryInterface.sequelize.getDialect();

    if (dialectName === "mysql") {
      await queryInterface.createTable("users", {
        id: {
          type: Sequelize.INTEGER,
          primaryKey: true,
          autoIncrement: true,
        },
        data: {
          type: Sequelize.JSON, // MySQL JSON type
        },
      });
    } else if (dialectName === "postgres") {
      await queryInterface.createTable("users", {
        id: {
          type: Sequelize.INTEGER,
          primaryKey: true,
          autoIncrement: true,
        },
        data: {
          type: Sequelize.JSONB, // PostgreSQL JSONB type
        },
      });
    }
  },
};

Resources

• references/orm-switching.md: ORM migration guides
• references/schema-migration.md: Schema transformation patterns
• references/data-transformation.md: Data migration scripts
• references/rollback-strategies.md: Rollback procedures
• assets/schema-migration-template.sql: SQL migration templates
• assets/data-migration-script.py: Data migration utilities
• scripts/test-migration.sh: Migration testing script

Best Practices

1. Always Provide Rollback: Every up() needs a down()
2. Test Migrations: Test on staging first
3. Use Transactions: Atomic migrations when possible
4. Backup First: Always backup before migration
5. Small Changes: Break into small, incremental steps
6. Monitor: Watch for errors during deployment
7. Document: Explain why and how
8. Idempotent: Migrations should be rerunnable

Common Pitfalls

• Not testing rollback procedures
• Making breaking changes without downtime strategy
• Forgetting to handle NULL values
• Not considering index performance
• Ignoring foreign key constraints
• Migrating too much data at once