========== Migrations ========== .. module:: django.db.migrations :synopsis: Schema migration support for Django models .. versionadded:: 1.7 Migrations are Django's way of propagating changes you make to your models (adding a field, deleting a model, etc.) into your database schema. They're designed to be mostly automatic, but you'll need to know when to make migrations, when to run them, and the common problems you might run into. A Brief History --------------- Prior to version 1.7, Django only supported adding new models to the database; it was not possible to alter or remove existing models via the ``syncdb`` command (the predecessor to ``migrate``). Third-party tools, most notably `South `_, provided support for these additional types of change, but it was considered important enough that support was brought into core Django. Two Commands ------------ There are two commands which you will use to interact with migrations and Django's handling of database schema: * :djadmin:`migrate`, which is responsible for applying migrations, as well as unapplying and listing their status. * :djadmin:`makemigrations`, which is responsible for creating new migrations based on the changes you have made to your models. It's worth noting that migrations are created and run on a per-app basis. In particular, it's possible to have apps that *do not use migrations* (these are referred to as "unmigrated" apps) - these apps will instead mimic the legacy behaviour of just adding new models. You should think of migrations as a version control system for your database schema. ``makemigrations`` is responsible for packaging up your model changes into individual migration files - analagous to commits - and ``migrate`` is responsible for applying those to your database. The migration files for each app live in a "migrations" directory inside of that app, and are designed to be committed to, and distributed as part of, its codebase. You should be making them once on your development machine and then running the same migrations on your colleagues' machines, your staging machines and eventually your production machines. Migrations will run the same way every time and produce consistent results, meaning that what you see in development and staging is exactly what will happen in production - no unexpected surprises. Backend Support --------------- Migrations are supported on all backends that Django ships with, as well as any third-party backends if they have programmed in support for schema alteration (done via the SchemaEditor class). However, some databases are more capable than others when it comes to schema migrations; some of the caveats are covered below. PostgreSQL ~~~~~~~~~~ PostgreSQL is the most capable of all the databases here in terms of schema support; the only caveat is that adding columns with default values will lock a table for a time proportional to the number of rows in it. For this reason, it's recommended you always create new columns with ``null=True``, as this way they will be added immediately. MySQL ~~~~~ MySQL lacks support for transactions around schema alteration operations, meaning that if a migration fails to apply you will have to manually unpick the changes in order to try again (it's impossible to roll back to an earlier point). In addition, MySQL will lock tables for almost every schema operation and generally takes a time proportional to the number of rows in the table to add or remove columns. On slower hardware this can be worse than a minute per million rows - adding a few columns to a table with just a few million rows could lock your site up for over ten minutes. Finally, MySQL has reasonably small limits on name lengths for columns, tables and indexes, as well as a limit on the combined size of all columns an index covers. This means that indexes that are possible on other backends will fail to be created under MySQL. SQLite ~~~~~~ SQLite has very little built-in schema alteration support, and so Django attempts to emulate it by: * Creating a new table with the new schema * Copying the data across * Dropping the old table * Renaming the new table to match the original name This process generally works well, but it can be slow and occasionally buggy. It is not recommended that you run and migrate SQLite in a production environment unless you are very aware of the risks and its limitations; the support Django ships with is designed to allow developers to use SQLite on their local machines to develop less complex Django projects without the need for a full database.