Understanding Sequelize database migrations and seeds

In my experience migrations change structure. Seeders... seed data. Recently I was on a project that didn't have seeders configured. https://sequelize.org/master/manual/migrations.html#seed-storage. This will allow you to setup a file so your data isn't seeded more than once. Migration configuration is right there as well.

In my opinion, a seeder is something, that is intended to run only once, while the migration is something that you add layer by layer to your DB structure continuously.

I would use seeders for populating some lookups or other data that most likely is not going to change, or test data. In the sequelize docs it's said, that "Seed files are some change in data that can be used to populate database table with sample data or test data."

If you want to make some dynamic data updates when data structure has already changed, you can run raw queries directly in your migrations if needed. So, if you, for instance, added some column in the up method, you can update the rows in the DB according to your business logic, e.g.:

// Add column, allow null at first
await queryInterface.addColumn("users", "user_type", {
    type: Sequelize.STRING,
    allowNull: true
});

// Update data
await queryInterface.sequelize.query("UPDATE users SET user_type = 'simple_user' WHERE is_deleted = 0;");

// Change column, disallow null
await queryInterface.changeColumn("users", "user_type", {
    type: Sequelize.STRING,
    allowNull: false
});

There is also an interesting discussion on this topic in Google Groups. I hope, this helps you.