Postgres failure when trying to remove check constraint

[Alan Evangelista]

Hi, everyone.

I have the following db model:

class Template(models.Model):

    max_documents = models.PositiveIntegerField(default=1)
    is_comparison = models.BooleanField(default=False)

    ...

    class Meta:

        constraints = [
            models.CheckConstraint(
                check=models.Q(is_comparison=False) | models.Q(max_documents__gte=2, is_comparison=True),
                name="is_comparison_requires_min_documents",
            ),
        ]

I recently added this check constraint above and makemigrations created the following db migration: 

class Migration(migrations.Migration):
    dependencies = [
        ...
    ]

    operations = [
        migrations.AddConstraint(
            model_name="template",
            constraint=models.CheckConstraint(
                check=models.Q(
                    ("is_comparison", False),
                    models.Q(("is_comparison", True), ("max_documents__gte", 2)),
                    _connector="OR",
                ),
                name="is_comparison_requires_min_documents",
            ),
        ),
    ]

Today I deleted the constraint and ran makemigrations again, which created this second migration:

class Migration(migrations.Migration):
    dependencies = [
        ...

    ]

    operations = [
        migrations.RemoveConstraint(
            model_name="template",
            name="is_comparison_requires_min_documents",
        ),
    ]

When I try to run the 2nd migration, I get the following Postgres error from Django:

django.db.utils.ProgrammingError: constraint "is_comparison_requires_min_documents" of relation "ai_template" does not exist

I checked the ai_template table schema and confirmed there is no check constraint there. It seems to me Django didn't create the check constraint on the Postgres db side (maybe it was enforcing it on its side) and now is trying to remove it from Postgres. Is that a bug or am I missing something?

Thanks in advance!

[Alan Evangelista]

I found out the cause was that I recently removed the max_documents field in the Template model, Postgres automatically deletes a constraint when one of the related columns is deleted and Django does not realize it. I manually updated the Django migration that deleted the field to also deleted the constrain and it worked fine.

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[Sam Brown]

For this exact reason I always use `sqlmigrate` to get the auto generated sql statement (and the backwards sql) and put this statements in  `forwards` and `backwards` functions. I then separate the db and state operations. 

I bet if you run `python manage.py sqlmigrate <app name> <migration>` on the migrations that removes the field from the model, you will see the sql that deletes the index. 

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