Only one registration for you

events/models.py

@@ -46,6 +46,15 @@ class EventRegistration(models.Model):
     user = models.ForeignKey(CustomUser, on_delete=models.CASCADE)
     state = models.CharField(max_length=1, choices=REGISTRATION_STATE.items())
 
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                fields=["event", "user"], name="user can only register once per event"
+            )
+        ]
+
+
     def __str__(self):
         return f"Reservation[{self.user.username}:{self.event.date}:{self.state}]"