Everybody Loves Migrations

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Andrew Godwin @andrewgodwin everybody loves migrations

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Django core developer Andrew Godwin South author Database wrangler extraordinare

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What is schema alteration? Background Why wasn't it in Django before? Why are you doing all this anyway?

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Adding a field What is schema alteration? Changing unique_together Setting db_index=True Changing the type of a field Renaming a field Setting null=True Adding a model Deleting a model

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Adding a field What Django did before Changing unique_together Setting db_index=True Changing the type of a field Renaming a field Setting null=True Adding a model Deleting a model

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Why is it so hard? Versioning Different databases Not necessary up front

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South

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Released 2008 Most popular solution Not without issues

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Basic Layout

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Basic Layout schemamigration: Creates migration files in apps

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Basic Layout schemamigration: Creates migration files in apps --auto: No, really, make them for me.

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Basic Layout schemamigration: Creates migration files in apps datamigration: Also creates migration files in apps --auto: No, really, make them for me.

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Basic Layout schemamigration: Creates migration files in apps datamigration: Also creates migration files in apps migrate: Applies migrations only --auto: No, really, make them for me.

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Basic Layout schemamigration: Creates migration files in apps datamigration: Also creates migration files in apps migrate: Applies migrations only syncdb: Does no-migration apps. --auto: No, really, make them for me.

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Issues Migrations build up over time VCS merges suck That file format...

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Migration actions Frozen ORM

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django.db.migrations A rewrite five years in the making

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Design Goals

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Clean migrations Readable diffs are really important

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Squashable migrations No need for those hundreds of old ones

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Better merge detection Because most teams don't read commits

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Better commands --auto is just so pointless now

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Automatic dependencies Stops those silent errors a year later

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Reuseable Schema API There are valid reasons to change tables

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Third-party compatability If nobody uses it, it's pointless

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Third-party compatability Squashable migrations Better merge detection Better commands Automatic dependencies Reuseable Schema API Clean migrations

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Design Overview

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Schema alteration Migration engine

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Schema alteration with connection.schema_editor() as editor: editor.create_model(Foo) editor.delete_field(Bar, "baz")

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Model and Field based Will eventually replace .creation

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Migrations makemigrations: Makes migrations. migrate: Applies migrations + legacy creation

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Migrations makemigrations: Makes migrations. migrate: Applies migrations + legacy creation syncdb: Gone!

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Autodetector Executor Makes new migrations Plans and runs migrations

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Better format

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Compound History Create Model Page with fields content, date Create Model Author with fields name, age Add title to Page Remove age from Author Migration 1 Migration 2 Migration 3 Model Page with fields title, content, date Model Author with field name

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Operations CreateModel DeleteModel AlterModelTable AlterUniqueTogether AlterIndexTogether AddField AlterField RenameField RemoveField RunSQL RunPython

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BONUS CODE

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Field.deconstruct() Nicer way of serialising fields Vital to both this and the upcoming composite fields project

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MORE BONUS CODE

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Multiple AppCaches If you know what this means, well done. Basically, you can make multiple versions of the same model in memory at once.

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Progress Report Where am I spending those Kickstarter pounds?

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Schema backends PostgreSQL MySQL SQLite Oracle Migration Engine Dependency resolver Basic operations makemigrations migrate squashmigrations DONE DONE DONE DONE DONE DONE DONE DONE NOT YET Advanced operations NOT YET Autodetector PARTIAL

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How does it affect YOU?

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Don't Panic! syncdb and signals still work Upgrade path from South migrations South 2 to backport new format

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Don't Panic! syncdb and signals still work Upgrade path from South migrations South 2 to backport new format Slightly Panic! Custom fields will need some work

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Future Ideas

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Better autodetection Automatic data migrations?

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Versioned model interface True downtimeless deployments?

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Percona support I almost feel sorry for the MySQL users.

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Nonrelational support Oh, they still need migrations.

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Things to Remember

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New migration format

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New migration format Even easier to use

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New migration format Django models through and through Even easier to use

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New migration format Django models through and through Please use a proper database Even easier to use

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New migration format Django models through and through Please use a proper database Schemas are your friend. Even easier to use

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Thanks. Questions welcome, especially from почемучка Andrew Godwin @andrewgodwin