Adding 'null=True' to a PointField doesn't create working migration

[Yaman Ozakin]

before:

class House(models.Model):
  point = models.PointField()

after:

class House(models.Model):
  point = models.PointField(null=True)

migration:

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.AlterField(
            model_name='house',
            name='point',
            field=django.contrib.gis.db.models.fields.PointField(null=True, srid=4326),
        ),
    ]

Traceback:

python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, myapp, sessions
Running migrations:
  Applying myapp.0002_auto_20180125_1119...Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/Basia/yamyam/research/django/myenv/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/Users/Basia/yamyam/research/django/myenv/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/Basia/yamyam/research/django/myenv/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/Basia/yamyam/research/django/myenv/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/Users/Basia/yamyam/research/django/myenv/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File "/Users/Basia/yamyam/research/django/myenv/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 "/Users/Basia/yamyam/research/django/myenv/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 "/Users/Basia/yamyam/research/django/myenv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/Basia/yamyam/research/django/myenv/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 "/Users/Basia/yamyam/research/django/myenv/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 "/Users/Basia/yamyam/research/django/myenv/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.House.point into myapp.House.point - they do not properly define db_type (are you using a badly-written custom field?)

Using Spatialite as backend engine.

[Jani Tiainen]

This is actually more widespread problem with spatialite backend.

DB geometry field in spatialite backend is defined as None which causes any kind of migration, even just adding verbose name which doesn't change field type nor anything database related to cause error message.

All other spatial backends seem to return at least some kind of a db field type.

[Victor Munene]

​PR