diff --git a/docs/topics/migrations.txt b/docs/topics/migrations.txt index afb56a9b28..a94ac0ede6 100644 --- a/docs/topics/migrations.txt +++ b/docs/topics/migrations.txt @@ -633,44 +633,42 @@ to be able to write your own, see the :doc:`migration operations reference `. -Optimizing migrations -===================== +Managing Migrations +=================== -Django developers are free to create as migrations as required and do not have -to worry about the number of migrations created as Django is optimized to deal -with hundreds at a time without much slowdown. However, there may come a time when migrations will become so numerous that it -will start to have a noticeable impact on various tasks like running tests on -CI. +You are encouraged to create as many migrations as required and do not have to +worry about the number of migrations created as Django is optimized to deal +with hundreds at a time without much slowdown. However, there may come a time +when migrations will become so numerous that it will start to have a noticeable +impact on various tasks like running tests on CI. There are a few steps that developers can take to mitigate or remediate this by reducing the amount of work that migrations has to do: - Optimizing migrations --------------------- -Django will automatically optimize migrations created via the -:djadmin:`makemigrations` command however you may also choose to optimize manually -created migrations with the :djadmin:`optimizemigration` command: +Migration operations can often be reduced to a more equivalent optimal form. An +example would be a :class:`~django.db.migrations.operations.CreateModel` +operation followed by a :class:`~django.db.migrations.operations.RenameModel` +operation: this can be reduced to a single operation by combining the final +name of the model with the +:class:`~django.db.migrations.operations.CreateModel`. + +Django automatically optimizes new migration files created with the +:djadmin:`makemigrations` command however you may also choose to optimize +manually created migration files with the :djadmin:`optimizemigration` command: .. code-block:: shell $ python manage.py optimizemigration -Migration optimization attempts to reduce the list of migration operations by -merging known pairs of possible reductions together. For example, it knows -that :class:`~django.db.migrations.operations.CreateModel` and -:class:`~django.db.migrations.operations.DeleteModel` cancel each other out, -and it knows that :class:`~django.db.migrations.operations.AddField` can be -rolled into :class:`~django.db.migrations.operations.CreateModel`. - - Updating migrations ------------------- -When creating new migrations you have the option of updating the previously -created migration if one exists. Running ``makemigrations --update`` will add any new operations to the last -migration created, then optimize it: +When making migrations you have the option of updating the previously created +migration if one exists. Running ``makemigrations --update`` will add any new +operations to the last migration created then optimize it: .. code-block:: shell @@ -681,17 +679,27 @@ migration created, then optimize it: Squashing migrations -------------------- -Squashing is the act of reducing an existing set of many migrations down to -one (or sometimes a few) migrations which still represent the same changes. +To optimize migrations across multiple migration files, "squashing" is +required. Squashing extracts all the operations from a set of migration files, +combines them together into an equivalent sequence, then optimizes the result. + +Squashed migrations + +Squashing can be done either automatically or manually, with both processes +sharing some steps. + +The steps are as follows: + + 1. Squash migrations into an optimal equivalent reduced set of migration files + 2. Set the ``replaces`` attribute -Using squashmigrations ----------------------- -Django has a tool to squash migrations automatically called :djadmin:`squashmigrations`. -This command takes all of your existing migrations, extracts their -``Operation``\s and puts them all in sequence, and then running the :ref:`optimizer ` -over them. +Automatically squashing migrations +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Django has a tool to squash migrations automatically called +:djadmin:`squashmigrations`. Once the operation sequence has been reduced as much as possible - the amount possible depends on how closely intertwined your models are and if you have @@ -751,7 +759,7 @@ brand new migrations from your models. In a future release of Django, itself. Manually squashing migrations ------------------------------ +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In this cases where :djadmin:`squashmigrations` is not able to run successfully, you can squash migrations manually using @@ -777,7 +785,6 @@ exception that the range must include the final migration: - Copy all non-elidable operation identified in step 2 into the newly created migration along with any dependencies. - Managing squashed migrations ----------------------------