Makemigrations raises KeyError when removing a field from Meta.unique_together when its a list

[Paco Martínez]

Having a model like this, with a unique_together with a list of lists:

from django.db import models

class MyModel(models.Model):
    field1 = models.CharField(max_length=50)
    field2 = models.IntegerField()
    field3 = models.DateField()

    class Meta:
        unique_together=[['field1', 'field2', 'field3']]

If we want to remove one of the fields from the constraint like:

    class Meta:
        unique_together=[['field1', 'field2']]

When we run the makemigrations command, it raises a KeyError. This is the traceback:

Traceback (most recent call last):
  File "/code/manage.py", line 26, in <module>
    main()
  File "/code/manage.py", line 22, in main
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.12/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.12/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.12/site-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.12/site-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/django/core/management/base.py", line 106, in wrapper
    res = handle_func(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/django/core/management/commands/makemigrations.py", line 233, in handle
    changes = autodetector.changes(
              ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/django/db/migrations/autodetector.py", line 46, in changes
    changes = self._detect_changes(convert_apps, graph)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/django/db/migrations/autodetector.py", line 192, in _detect_changes
    self.generate_removed_altered_unique_together()
  File "/usr/local/lib/python3.12/site-packages/django/db/migrations/autodetector.py", line 1506, in generate_removed_altered_unique_together
    self._generate_removed_altered_foo_together(operations.AlterUniqueTogether)
  File "/usr/local/lib/python3.12/site-packages/django/db/migrations/autodetector.py", line 1481, in _generate_removed_altered_foo_together
    for (
  File "/usr/local/lib/python3.12/site-packages/django/db/migrations/autodetector.py", line 1462, in _get_altered_foo_together_operations
    field = new_model_state.get_field(field_name)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/django/db/migrations/state.py", line 765, in get_field
    return self.fields[field_name]
           ~~~~~~~~~~~^^^^^^^^^^^^
KeyError: 'field1'

This does NOT happen when the unique_together option is a tuple. It works as expected creating the migration.