diff --git a/seminar/migrations/0065_treenode_polymorphic_ctype.py b/seminar/migrations/0065_treenode_polymorphic_ctype.py
index 71eef262..cb65d8f1 100644
--- a/seminar/migrations/0065_treenode_polymorphic_ctype.py
+++ b/seminar/migrations/0065_treenode_polymorphic_ctype.py
@@ -5,7 +5,7 @@ import django.db.models.deletion
 
 def vyrob_treenodum_ctypes(apps, schema_editor):
 	# Kód zkopírovaný z dokumentace: https://django-polymorphic.readthedocs.io/en/stable/migrating.html
-	# XXX: Nevím, jestli se tohle náhodou nemělo spustit na všech childech (jen/i)
+	# NOTE: Tahle migrace je špatně, 0087 ji opravuje. Možno squashnout pryč.
 	TreeNode = apps.get_model('seminar', 'TreeNode')
 	ContentType = apps.get_model('contenttypes', 'ContentType')
 	
@@ -27,5 +27,5 @@ class Migration(migrations.Migration):
 			name='polymorphic_ctype',
 			field=models.ForeignKey(editable=False, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='polymorphic_seminar.treenode_set+', to='contenttypes.ContentType'),
 		),
-		migrations.RunPython(vyrob_treenodum_ctypes, migrations.RunPython.noop),
+		migrations.RunPython(vyrob_treenodum_ctypes, migrations.RunPython.noop, elidable=True),
 	]
diff --git a/seminar/migrations/0066_problem_polymorphic_ctype.py b/seminar/migrations/0066_problem_polymorphic_ctype.py
index f956217e..3aab4605 100644
--- a/seminar/migrations/0066_problem_polymorphic_ctype.py
+++ b/seminar/migrations/0066_problem_polymorphic_ctype.py
@@ -5,7 +5,7 @@ import django.db.models.deletion
 
 def vyrob_problemum_ctypes(apps, schema_editor):
 	# Kód zkopírovaný z dokumentace: https://django-polymorphic.readthedocs.io/en/stable/migrating.html
-	# XXX: Nevím, jestli se tohle náhodou nemělo spustit na všech childech (jen/i)
+	# NOTE: Tahle migrace je špatně, 0087 ji opravuje. Možno squashnout pryč.
 	Problem = apps.get_model('seminar', 'Problem')
 	ContentType = apps.get_model('contenttypes', 'ContentType')
 	
@@ -25,5 +25,5 @@ class Migration(migrations.Migration):
 			name='polymorphic_ctype',
 			field=models.ForeignKey(editable=False, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='polymorphic_seminar.problem_set+', to='contenttypes.ContentType'),
 		),
-		migrations.RunPython(vyrob_problemum_ctypes, migrations.RunPython.noop),
+		migrations.RunPython(vyrob_problemum_ctypes, migrations.RunPython.noop, elidable=True),
 	]
diff --git a/seminar/migrations/0087_fix_polymorphism.py b/seminar/migrations/0087_fix_polymorphism.py
new file mode 100644
index 00000000..d38877ba
--- /dev/null
+++ b/seminar/migrations/0087_fix_polymorphism.py
@@ -0,0 +1,49 @@
+# Generated by Django 2.2.16 on 2020-09-04 12:06
+
+from django.db import migrations
+from logging import getLogger
+
+log = getLogger(__name__)
+
+# Oprava migrací 0065 a 0066, kde jsem špatně pochopil django-polymorphic
+
+# Pomocná funkce -- děláme to samé pro obě polymorfní hierarchie
+def fix_ctypes(parent: str, children, apps, schema_editor):
+	Parent = apps.get_model('seminar', parent)
+	ContentType = apps.get_model('contenttypes', 'ContentType')
+
+	# Nejdřív všechno smažeme:
+	Parent.objects.update(polymorphic_ctype=None)
+
+	# Opravíme děti
+	for clsname in children:
+		Model = apps.get_model('seminar', clsname)
+		ct = ContentType.objects.get_for_model(Model)
+		Model.objects.update(polymorphic_ctype=ct)
+	
+
+	# Ostatní instance mají mít explicitně content type pro rodiče
+	new_ct = ContentType.objects.get_for_model(Parent)
+	for obj in Parent.objects.filter(polymorphic_ctype__isnull=True):
+		log.warn(f"{parent} {obj} neměl content type -- nejspíš to je instance přímo {parent}!")
+		obj.polymorphic_ctype=new_ct
+		obj.save()
+
+def fix_treenode(apps, schema_editor):
+	children = ['RocnikNode', 'CisloNode', 'MezicisloNode', 'TemaVCisleNode',
+			'OrgTextNode', 'UlohaZadaniNode', 'UlohaVzorakNode', 'PohadkaNode',
+			'TextNode', 'CastNode', 'ReseniNode']
+	fix_ctypes("TreeNode", children, apps, schema_editor)
+
+def fix_problem(apps, schema_editor):
+	children = ['Tema', 'Clanek', 'Uloha', 'Konfera']
+	fix_ctypes("Problem", children, apps, schema_editor)
+
+class Migration(migrations.Migration):
+	dependencies = [
+		('seminar', '0086_auto_20200819_0959'),
+	]
+	operations = [
+		migrations.RunPython(fix_treenode, migrations.RunPython.noop),
+		migrations.RunPython(fix_problem, migrations.RunPython.noop),
+	]