TreeNode: first_child je 1-2-1-Field, ne FK.

seminar/migrations/0083_auto_20200506_1952.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.12 on 2020-05-06 17:52
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('seminar', '0082_auto_20200506_1951'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='treenode',
+            name='first_child',
+            field=models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='father_of_first', to='seminar.TreeNode', verbose_name='první potomek'),
+        ),
+    ]

seminar/models.py

@@ -1246,7 +1246,7 @@ class TreeNode(PolymorphicModel):
 		blank = False,
 		on_delete = models.SET_NULL, # Vrcholy s null kořenem jsou sirotci bez ročníku 
 		verbose_name="kořen stromu")
-	first_child = models.ForeignKey('TreeNode',
+	first_child = models.OneToOneField('TreeNode',
 		related_name='father_of_first',
 		null = True,
 		blank = True,