feat(venue): make sure we unique based on name, city, and state (when…

… present)

note that if state is NULL (or blank), you can still have duplicate venues as databases treat NULL values as distinct

concerts/models.py

@@ -42,7 +42,7 @@ class Venue(models.Model):
     created_at = models.DateTimeField(auto_now_add=True)
 
     class Meta:
-        unique_together = ["name", "city"]
+        constraints = [models.UniqueConstraint(fields=["name", "city", "state"], name="unique_venue")]
 
     def __str__(self):
         if self.country: