Migrate default edition field to work model

Fixes #281

bookwyrm/migrations/0008_work_default_edition.py

@@ -0,0 +1,35 @@
+# Generated by Django 3.0.7 on 2020-11-04 18:15
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+def set_default_edition(app_registry, schema_editor):
+    db_alias = schema_editor.connection.alias
+    works = app_registry.get_model('bookwyrm', 'Work').objects.using(db_alias)
+    editions = app_registry.get_model('bookwyrm', 'Edition').objects.using(db_alias)
+    for work in works:
+        ed = editions.filter(parent_work=work, default=True).first()
+        if not ed:
+            ed = editions.filter(parent_work=work).first()
+        work.default_edition = ed
+        work.save()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0007_auto_20201103_0014'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='work',
+            name='default_edition',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.PROTECT, to='bookwyrm.Edition'),
+        ),
+        migrations.RunPython(set_default_edition),
+        migrations.RemoveField(
+            model_name='edition',
+            name='default',
+        ),
+    ]