making the migration reversible

src/documents/migrations/1003_mime_types.py

@@ -1,4 +1,5 @@
 # Generated by Django 3.1.3 on 2020-11-20 11:21
+import mimetypes
 import os
 
 import magic
@@ -29,6 +30,15 @@ def add_mime_types(apps, schema_editor):
         d.save()
 
 
+def add_file_extensions(apps, schema_editor):
+    Document = apps.get_model("documents", "Document")
+    documents = Document.objects.all()
+
+    for d in documents:
+        d.file_type = os.path.splitext(d.filename)[1].strip('.')
+        d.save()
+
+
 class Migration(migrations.Migration):
 
     dependencies = [
@@ -42,7 +52,24 @@ class Migration(migrations.Migration):
             field=models.CharField(default="-", editable=False, max_length=256),
             preserve_default=False,
         ),
-        migrations.RunPython(add_mime_types),
+        migrations.RunPython(add_mime_types, migrations.RunPython.noop),
+
+        # This operation is here so that we can revert the entire migration:
+        # By allowing this field to be blank and null, we can revert the
+        # remove operation further down and the database won't complain about
+        # NOT NULL violations.
+        migrations.AlterField(
+            model_name='document',
+            name='file_type',
+            field=models.CharField(
+                choices=[('pdf', 'PDF'), ('png', 'PNG'), ('jpg', 'JPG'), ('gif', 'GIF'), ('tiff', 'TIFF'), ('txt', 'TXT'), ('csv', 'CSV'), ('md', 'MD')],
+                editable=False,
+                max_length=4,
+                null=True,
+                blank=True
+            ),
+        ),
+        migrations.RunPython(migrations.RunPython.noop, add_file_extensions),
         migrations.RemoveField(
             model_name='document',
             name='file_type',