Refactor Assignment model to enforce single rota assignment per worker and update related view logic to prevent multiple assignments

djangorota/rota/models.py

@@ -297,13 +297,18 @@ class Worker(models.Model):
 class Assignment(models.Model):
 	"""Join model assigning a worker to a rota (can hold role / metadata)."""
 
-	worker = models.ForeignKey(Worker, on_delete=models.CASCADE)
+	# Enforce that a Worker may only be assigned to a single RotaSchedule by
+	# using a OneToOneField here. This ensures at the DB level that each
+	# Worker has at most one Assignment (and therefore at most one rota).
+	worker = models.OneToOneField(Worker, on_delete=models.CASCADE)
 	rota = models.ForeignKey(RotaSchedule, on_delete=models.CASCADE)
 	role = models.CharField(max_length=100, blank=True)
 	created_at = models.DateTimeField(auto_now_add=True)
 
 	class Meta:
-		unique_together = ("worker", "rota")
+		# With OneToOneField on worker, uniqueness for (worker, rota) is
+		# implied by the worker uniqueness. Keep ordering for convenience.
+		ordering = ("-created_at",)
 
 	def __str__(self):
 		return f"{self.worker} -> {self.rota}"