{"id":422,"date":"2021-12-13T23:37:46","date_gmt":"2021-12-13T23:37:46","guid":{"rendered":"https:\/\/zerotobyte.com\/?p=422"},"modified":"2021-12-20T15:38:13","modified_gmt":"2021-12-20T15:38:13","slug":"django-data-migrations-how-to-change-data-in-the-database","status":"publish","type":"post","link":"https:\/\/zerotobyte.com\/django-data-migrations-how-to-change-data-in-the-database\/","title":{"rendered":"Django Data Migrations – How to Change Data in the Database"},"content":{"rendered":"\n

In this article, I\u2019ll explain what Django data migrations are and how to write custom database migrations. Django schema migrations are already covered in this article<\/a>. If you don\u2019t know the difference between data migrations and schema migrations, here\u2019s a quick explanation of both.<\/p>\n\n\n\n

Schema migrations are responsible only for making changes to the database structure<\/strong>. If you want to add a new column, remove the existing one, or define a new relationship between tables, you must apply schema migrations. With the help of makemigrations<\/code> and migrate<\/code> commands, Django propagates model changes to the database schema<\/strong>.<\/p>\n\n\n\n

Data migrations are used to make changes not to database structure but data residing in the database<\/strong>. Unlike schema migrations, data migrations are not generated by Django and must be defined manually<\/strong>. They define how the existing data will be altered or loaded in the database. Changes must be defined inside a callback function passed to the RunPython command. Executing the <\/strong>migrate<\/code> command will apply changes to data in the database<\/strong>.<\/p>\n\n\n\n

How Django data migrations work<\/h2>\n\n\n\n

Aside from being different types of migrations, data and schema migrations work similarly. Both are defined inside migration files in the app\u2019s migrations directory. Also, both represent changes as operations that need to be executed sequentially in the exact order as specified. Those changes are listed in the operations attribute of a Django Migration class.<\/p>\n\n\n\n

A quick reminder: the migration file contains a Migration class with the dependencies and operations attributes<\/strong>. Previous migrations that the current one is dependent on are contained in the dependencies<\/code> list. The operations<\/code> list defines operations that are first translated from Python code to SQL statements, then executed in the database.<\/p>\n\n\n\n

from django.db import migrations\n\nclass Migration(migrations.Migration):\n    dependencies = [\n    ]\n\n    operations = [\n    ]<\/pre>\n\n\n\n
Note that the usual order of making migrations is first to apply schema migrations, then optionally data migrations if needed. While schema migrations are generated automatically by Django in most cases, data migrations need to be defined manually<\/strong>.<\/p>\n\n\n\n
The main difference between those two types of migrations is that data migrations are defined as a custom Python code and passed to the RunPython operation. After all changes are listed, the migrate<\/code> command is responsible for applying those changes to the database.<\/p>\n\n\n\n
If you are already familiar with the Django schema migrations, you\u2019ll master data migrations quickly as well. I\u2019ll explain now the RunPython operation in more detail.<\/p>\n\n\n\n
RunPython method<\/h3>\n\n\n\n
As mentioned above, changes meant to be applied to data in the database are defined as custom Python code. RunPython method is responsible for handling custom migration via callable objects passed to it<\/strong>. Actually, RunPython method accepts two callable objects (code<\/code>, reverse_code<\/code>), also named forward and reverse functions. <\/p>\n\n\n\n