GeoDjango MultiPointField Dimension Migration Issue

You've encountered a known issue with GeoDjango's migration system when changing the dimension of a MultiPointField. This appears to be a bug in how Django's migration framework handles dimension changes for spatial fields.

Analysis of the Issue

The problem occurs because:

1. When you change the dim parameter from 2 to 3, Django's migration detection system doesn't properly recognize this as a field change that requires a schema modification.

2. The generated migration doesn't include the dim=3 parameter in the AlterField operation, which means the database schema won't be properly updated to accommodate 3D points.

3. The error message Cannot alter field... they do not properly define db_type suggests that Django's migration system is detecting a mismatch between the field definitions but isn't able to generate the correct SQL to transform the field.

Workarounds

There are a few approaches to handle this issue:

Option 1: Manual Migration

Create a custom migration that explicitly handles the dimension change:

PYTHONCopied
operations = [
    migrations.AlterField(
        model_name='multipointtester',
        name='multipoint',
        field=django.contrib.gis.db.models.fields.MultiPointField(
            dim=3,  # Explicitly include the dimension
            blank=True, 
            default='SRID=4326;MULTIPOINT EMPTY', 
            null=True, 
            srid=4326
        ),
    ),
]

Option 2: Two-Step Migration

1. First, create a migration that removes the field
2. Then create a second migration that adds the field back with the new dimension

Option 3: Reset Migrations (as you discovered)

This is what you've already found works - removing all migrations and starting fresh. This is a valid approach for development but may not be feasible in production environments.

Best Practices for GeoDjango Dimension Changes

When working with GeoDjango spatial fields:

1. Try to determine the correct dimension (2D, 3D) at the beginning of your project
2. If you need to change dimensions, consider creating a new field and migrating data rather than altering the existing field
3. Test dimension changes thoroughly in a development environment before applying to production

This issue is related to how GeoDjango handles spatial transformations. As noted in the documentation, GeoDjango has specific handling for coordinate systems and transformations, but the migration system doesn't seem to fully account for dimension changes.

If you continue to experience issues, you might want to consider reporting this as a bug to the Django project, as it appears to be a limitation in the current migration framework's handling of spatial field dimensions.