docs: def migrate_flow_collection(apps, schema_editor):
      """Migrate 'flow_collection' field to 'entity_type'."""
      Process = apps.get_model('flow', 'Process')
      DescriptorSchema = apps.get_model('flow', 'DescriptorSchema')
  
      for process in Process.objects.all():
          process.entity_type = process.flow_collection
          process.entity_descriptor_schema = process.flow_collection
  
          if (process.entity_descriptor_schema is not None and
                  not DescriptorSchema.objects.filter(slug=process.entity_descriptor_schema).exists()):
              raise LookupError(
                  "Descriptow schema '{}' referenced in 'entity_descriptor_schema' not "
                  "found.".format(process.entity_descriptor_schema)
              )
  
          process.save()