Python 2 -> 3 Django migration causes field parameter type change

Question

We are transitioning a Django project from Django 1.8 -> 2.1 and Python 2.7 -> 3.6.

In the old project version, there are Django models that looked like this, for example:

# models.py

from django.db import models

class RowStatusModel(models.Model):
    active = models.BooleanField(default=True, db_column='is_active')
    # ...
    class Meta:
        abstract = True

Notice that from __future__ import unicode_literals is not used in this module. That means that db_column is a Python 2 str, corresponding to bytes in Python 3. The initial migration, 0001_initial.py, looks like this:

# 0001_initial.py

operations = [
    # ...
    ('row_ef', models.BooleanField(default=True, db_column=b'is_active')
    # ...
]

Notice the byte-literal b'is_active, which I suppose was done by Django in the interest of being more explicit, but am not sure.

Now after transitioning most of the codebase with 2to3 and running makemigrations, Python 3 treats a string literal as what would be the unicode type in Python 2, and consequently generates a migration where the db_column is a string literal, for every single model that inherits from RowStatusModel:

# migrations/0023_auto_20180827_1955.py 
migrations.AlterField(
    # ...
    field=models.BooleanField(default=True, db_column='is_active')
), # ...

What effect, if any, will this have on the database side when ./manage.py migrate is ran? Is the "change" purely on the Python side, or what side effects may be created?

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Database engine is django.db.backends.postgresql.

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I'm aware that we can just clone the RDS instance and revert back to that if migrate causes immediate problems, but I am more concerned about more subtle issues being introduced that would not be caught until much later on.

Answer 1

I came across a Django ticket after asking this question, where the recommendation from a Django developer is to edit any legacy migrations files (such as 0001_initial) that contain (Python 3) bytes literals, removing the b and making them string-literals in Python 3.

Editing migrations to fix this issue is safe.

From there you should presumably be able to delete the migrations modules that were made with python3 ./manage.py makemigrations, and redo that command, which should no longer take issue with the type of that argument.

I used the following to do a mass-find-and-replace of all instances of 'db_column=b' with 'db_column='. Granted, you should absolutely check git diff before making that commit.

grep -nrl "db_column=b" apps | xargs sed -i "s/db_column=b/db_column=/g"