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As per the django docs when creating django migrations we should use apps.get_model() rather than importing the models and using them.

Why does a data migration have to use the historical version of a model rather than the latest one?(The historical versions of the model will not be in use anyways right?)

user3282666
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2 Answers2

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It uses the historical versions of the model so that it won't have problems trying to access fields that may no longer exist in the code base when you run your migrations against another database.

If you removed some field from your model and then wanted to run your migrations on some new database, and you were importing your models directly, you can expect your migrations would complain trying to use a field that doesn't exist. When using apps.get_model(...) Django will try to be smart about it and use the definitions of migrations.AddField(...) from your migrations files to give you the correct version of your model at that point in time.

This is also why Django says to be careful about using custom Model/Model Manager methods in your data migrations because I don't believe they can recreate these methods from the migrations history, or the behaviour can change over time and your migrations wouldn't be consistent.

A. J. Parr
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    If i remove a field in a model, I would want my migration to focus only on the fields that exist and not on the field removed. So when django complains that I am trying to use a field that does not exist, that is want we expect django to complain right? – user3282666 Jun 12 '16 at 06:19
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    Django could be talking about a data migration written 6 months ago when the models were completely different, but that data migration still needs to work the same now as it did 6 months ago. It shouldn't complain about a field missing in your current model because the data migration wasn't designed for the current model, its designed for a specific version of your model. – A. J. Parr Jun 15 '16 at 04:49
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    Thanks! So dumb that I could not think of this scenario – user3282666 Jun 16 '16 at 08:02
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Consider this model:

class A(models.Model):
    field1 = models.PositiveIntegerField()
    field2 = models.PositiveIntegerField()

Your migration history knows about these two fields and any further migration will consider this model state and will make changes to this model state.

Now suppose, you remove field1 and your model becomes:

class A(models.Model):
    field2 = models.PositiveIntegerField()

And in the migration, you try to use field1, django should know that field1 existed. Hence when we use apps.get_model(), it helps django use the previous migrations history and infer about field1. Otherwise you will get an error.

rohanagarwal
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