Changing SRID on geometry field doesn't create working migration

[Jani Tiainen]

When changing SRID of geometry field:

before:

class AddressLog(models.Model):
    location = models.PointField(blank=True, null=True, srid=4326)

after:

class AddressLog(models.Model):
    location = models.PointField(blank=True, null=True, srid=3067)

Migration:

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0002_auto_20171114_1021'),
    ]

    operations = [
        migrations.AlterField(
            model_name='addresslog',
            name='location',
            field=django.contrib.gis.db.models.fields.PointField(blank=True, null=True, srid=3067),
        ),
    ]

Traceback:

(myapp) jtiai@jns42-l:~/projects/myproject (default) $ ./manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, sessions, myapp
Running migrations:
  Applying myapp.0003_auto_20171117_0641...Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/db/migrations/executor.py", line 115, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 216, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/home/jtiai/.virtualenvs/myapp/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 494, in alter_field
    (old_field, new_field),
ValueError: Cannot alter field myapp.AddressLog.location into myapp.AddressLog.location - they do not properly define db_type (are you using a badly-written custom field?)

Using Spatialite as backend engine.

[Claude Paroz]

Changing the SRID of a field is something that's hard to do when you have existing content. I don't think databases are doing anything clever when you do that (e.g. they don't convert the existing values, AFAIK). Probably the best strategy is to create a new column, move the values, converting them in the new SRID in the process, and delete the old column. The strategy might differ depending on the backend.

[Jani Tiainen]

True, SRID change migrations is not easy to do (might not even be possible sometimes due missing conversion parameters). But case should be handled much more gracefully than throwing out traceback.