Changing violation_error_message on constraints cause a remove/add operation in migration

[David]

Changing the violation_error_message of a constraint and running makemigrations causes the migration to be written with two operations RemoveConstraint and AddConstraint .

Suppose we start with this situation:

from django.db import models

class MyModel(models.Model):
    counter = models.IntegerField(defalt=0)
    max_value = models.IntegerField(defalt=0)
    name = models.CharField(max_length=4)

    class Meta:
        constraints = (
            models.CheckConstraint(
                check=models.Q(counter__gte=models.F('max_value')),
                name='counter_lower_than_max',
            ),
            models.UniqueConstraint(
                fields=('name', 'max_value')
                name='uniq_name_and_max',
            ),
        )

If we add a custom violation_error_message and run makemigrations it will output

$ ./manage.py makemigrations mysample -n update_violation_msgs
Migrations for 'mysample':
  mysample/migrations/0002_update_violation_msgs.py
    - Remove constraint counter_lower_than_max from model mymodel
    - Create constraint counter_lower_than_max on model mymodel
    - Remove constraint uniq_name_and_maxfrom model mymodel
    - Create constraint uniq_name_and_maxon model mymodel

This will cause the database tu run useless commands, since nothing on database side has changed!
This may be a particular problem when applied to UniqueConstraints since the database needs to destroy and re-create an index.

[David Sanders]

Thanks for the report 🏆

Aside from running useless commands, dropping & adding constraints on a production database may be undesirable.

[Salvo Polizzi]

PR: ​https://github.com/django/django/pull/17644

[Mariusz Felisiak]

Marking as needs improvement pending a discussion.