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

djangorota/rota/management/commands/find_multiple_assignments.py

@@ -0,0 +1,16 @@
+from django.core.management.base import BaseCommand
+from django.db.models import Count
+
+from rota.models import Worker
+
+
+class Command(BaseCommand):
+    help = "List workers assigned to more than one rota (useful before migrating to OneToOne Assignment)"
+
+    def handle(self, *args, **options):
+        qs = Worker.objects.annotate(num_rotas=Count('rotas')).filter(num_rotas__gt=1)
+        if not qs.exists():
+            self.stdout.write(self.style.SUCCESS('No workers assigned to more than one rota.'))
+            return
+        for w in qs:
+            self.stdout.write(f"Worker id={w.id} name={w.name!r} assigned_rotas={w.num_rotas}")

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}"

djangorota/rota/views.py

@@ -334,12 +334,19 @@ def worker_create(request):
 				# If we created a worker and a rota_id was provided, create Assignment
 				if rota is not None:
 					from .models import Assignment
+					# Only assign if the worker is not already assigned to any rota.
 					try:
-						Assignment.objects.create(worker=worker, rota=rota)
+						if not worker.rotas.exists():
+							Assignment.objects.create(worker=worker, rota=rota)
+						else:
+							# skip assigning; worker already assigned elsewhere
+							pass
 					except Exception:
-						# best-effort: try using the M2M add as fallback
+						# best-effort: try using the M2M add as fallback but still
+						# respect single-rota constraint
 						try:
-							rota.workers.add(worker)
+							if not worker.rotas.exists():
+								rota.workers.add(worker)
 						except Exception:
 							pass
 
@@ -454,14 +461,37 @@ def worker_import(request, rota_id=None):
 				except Exception:
 					fte = 1.0
 
-				w = Worker.objects.create(name=name.strip(), email=email.strip(), site=site.strip(), grade=grade, fte=fte)
+				# If an email is provided, try to find an existing worker to
+				# avoid duplicates; otherwise create a new record.
+				existing = None
+				if email:
+					try:
+						existing = Worker.objects.filter(email__iexact=email.strip()).first()
+					except Exception:
+						existing = None
+
+				if existing is not None:
+					w = existing
+				else:
+					w = Worker.objects.create(name=name.strip(), email=email.strip(), site=site.strip(), grade=grade, fte=fte)
 				created.append(w)
+
+				# Assignment: only assign if the worker is not already assigned
+				# to another rota. If already assigned, record an error and
+				# skip assignment.
 				if assign_to_rota and rota is not None:
 					try:
-						Assignment.objects.create(worker=w, rota=rota)
+						if not w.rotas.exists():
+							Assignment.objects.create(worker=w, rota=rota)
+						else:
+							# If already assigned to this rota, nothing to do.
+							assigned_rota = w.rotas.first()
+							if assigned_rota and assigned_rota.pk != rota.pk:
+								errors.append(f"Row {i+1}: worker '{w.name}' already assigned to rota '{assigned_rota.name}'")
 					except Exception:
 						try:
-							rota.workers.add(w)
+							if not w.rotas.exists():
+								rota.workers.add(w)
 						except Exception:
 							pass
 			except Exception as exc: