The answer
When you add, remove, or rename fields. When you add indexes. When you change data types. Any schema change that affects existing data needs a migration, so the old data works with the new code.
Database Migrations and Versioning in Node.js Apps
When do you need a database migration?
More FAQs in Database Migrations and Versioning in Node.js Apps
Scripts that modify the database schema (add tables, columns, indexes) in a controlled, reversible way. They let you evolve the schema without manual database changes, and are version-controlled alongside code.
Each migration has up (apply) and down (revert) scripts, run in order. Tools like Knex.js and Sequelize migrations handle this for Node.js. The tool tracks which migrations have been applied so each environment is in sync.
NoSQL is more flexible, but data transformations need migrations. Use tools like migrate-mongo or write custom scripts that transform existing documents when the schema changes, ensuring existing data works with new code.
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