diff --git a/seminar/models.py b/seminar/models.py
index 57d4304a..43d6e59d 100644
--- a/seminar/models.py
+++ b/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,