How migrations work?

Once you get into production you'll need to synchronize model changes into the database. Typically, it is unsafe to use synchronize: true for schema synchronization on production once you get data in your database. Here is where migrations come to help.

A migration is just a single file with SQL queries to update a database schema and apply new changes to an existing database.

Let's say you already have a database and a Post entity:

import { Entity, Column, PrimaryGeneratedColumn } from "typeorm"

@Entity()
export class Post {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    text: string
}

And your entity worked in production for months without any changes. You have thousands of posts in your database.

Now you need to make a new release and rename title to name. What would you do?

You need to create a new migration with the following SQL query (PostgreSQL dialect):

ALTER TABLE "post" RENAME COLUMN "title" TO "name";

Once you run this SQL query your database schema is ready to work with your new codebase. TypeORM provides a place where you can write such sql queries and run them when needed. This place is called "migrations".