How to ignore a specific migration?

Question

I have a migration like this:

class Migration(migrations.Migration):

    dependencies = [
        ('app', '0020_auto_20191023_2245'),
    ]

    operations = [
        migrations.AddField(
            model_name='agenda',
            name='theme',
            field=models.PositiveIntegerField(default=1),
        ),
    ]

But it raises an error:

django.db.utils.ProgrammingError: column "theme" of relation "app_agenda" already exists

Not a problem, I've wrapped this error like this:

from django.db import migrations, models, ProgrammingError


def add_field_theme_to_agenda(apps, schema_editor):
    try:
        migrations.AddField(
            model_name='agenda',
            name='theme',
            field=models.PositiveIntegerField(default=1),
        ),
    except ProgrammingError as e:  # sometimes it can exist
        if "already exists" not in str(e):
            raise


class Migration(migrations.Migration):
    dependencies = [
        ('app', '0020_auto_20191023_2245'),
    ]
    operations = [
        migrations.RunPython(add_field_theme_to_agenda),
    ]

This works like a charm and all the following migrations are done.

My problem is that each time I run a "makemigrations" Django adds again the migration (= the one on the top of my question). I guess it's because it doesn't see it in the migrations, because my code obfuscate it.

How to circumvent this using migrations (dont say answers like "this problem is on your database, correct your database")?

Answer 1

Django is re-creating the migrations because as you are doing the operation manually inside a RunPython operation, it can not understand the field is added. What you can try is (haven't tried this myself), subclass AddField operations to create a custom AddField operation, where you can handle the exception. Something like the following could work:

from django.db import migrations, models, ProgrammingError


class AddFieldIfNotExists(migrations.AddField):

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        try:
            super().database_forwards(app_label, schema_editor, from_state,
                                      to_state)
        except ProgrammingError as e:  # sometimes it can exist
            if "already exists" not in str(e):
                raise


class Migration(migrations.Migration):
    atomic = False
    dependencies = [
        ('app', '0070_auto_20191023_1203'),
    ]

    operations = [
        AddFieldIfNotExists(
            model_name='agenda',
            name='theme',
            field=models.PositiveIntegerField(default=1),
        ),
    ]