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Add clarifying section about historical migration models

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Andrew Godwin 2018-05-15 12:19:30 -04:00 committed by Andrew Godwin
parent a177f854c3
commit 0a8a6b4683
1 changed files with 13 additions and 2 deletions
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15
docs/topics/migrations.txt
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@ -352,8 +352,19 @@ Historical models
When you run migrations, Django is working from historical versions of your
models stored in the migration files. If you write Python code using the
:class:`~django.db.migrations.operations.RunPython` operation, or if you have
``allow_migrate`` methods on your database routers, you will be exposed to
these versions of your models.
``allow_migrate`` methods on your database routers, you **need to use** these
historical model versions rather than importing them directly.
.. warning::
If you import models directly rather than using the historical models,
your migrations *may work initially* but will fail in the future when you
try to re-run old migrations (commonly, when you set up a new installation
and run through all the migrations to set up the database).
This means that historical model problems may not be immediately obvious.
If you run into this kind of failure, it's OK to edit the migration to use
the historical models rather than direct imports and commit those changes.
Because it's impossible to serialize arbitrary Python code, these historical
models will not have any custom methods that you have defined. They will,