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