add a reverse migration

arkindex/documents/migrations/0030_remove_entity_metatype.py

 # Generated by Django 2.2 on 2019-12-13 10:28
 
 from django.db import migrations
+from arkindex_common.enums import EntityType
 from enum import Enum
 
 
 def remove_entity_metatype(apps, schema_editor):
     MetaData = apps.get_model('documents', 'MetaData')
+    if not MetaData.objects.exists():
+        return
 
     class OldMetaType(Enum):
         Text = 'text'
@@ -15,17 +18,25 @@ def remove_entity_metatype(apps, schema_editor):
         Reference = 'reference'
         Entity = 'entity'
 
-    values = set(item.value for item in OldMetaType)
-
     # Don't try this at home
     MetaData._meta.fields[2].enum = OldMetaType
 
-    for metadata in MetaData.objects.filter(type=OldMetaType.Entity).prefetch_related('entity'):
-        metadata_type = OldMetaType.Text
-        if metadata.entity and metadata.entity.type.value in values:
-            metadata_type = OldMetaType(metadata.entity.type.value)
-        metadata.type = metadata_type
-        metadata.save()
+    MetaData.objects.filter(type=OldMetaType.Entity, entity__type=EntityType.Location).update(type=OldMetaType.Location)
+    MetaData.objects.filter(type=OldMetaType.Entity, entity__type=EntityType.Date).update(type=OldMetaType.Date)
+    MetaData.objects.filter(type=OldMetaType.Entity).update(type=OldMetaType.Text)
+
+
+def add_entity_metatype(apps, schema_editor):
+    MetaData = apps.get_model('documents', 'MetaData')
+    if not MetaData.objects.exists():
+        return
+
+    class OldMetaType(Enum):
+        Entity = 'entity'
+
+    # Don't try this at home
+    MetaData._meta.fields[2].enum = OldMetaType
+    MetaData.objects.exclude(entity__isnull=True).update(type=OldMetaType.Entity)
 
 
 class Migration(migrations.Migration):
@@ -35,8 +46,5 @@ class Migration(migrations.Migration):
     ]
 
     operations = [
-        migrations.RunPython(
-            remove_entity_metatype,
-            reverse_code=migrations.RunPython.noop
-        ),
+        migrations.RunPython(remove_entity_metatype, add_entity_metatype),
     ]