Add the condition to the serializer

arkindex/training/serializers.py

@@ -4,7 +4,7 @@ from collections import defaultdict
 from textwrap import dedent
 
 from django.db import transaction
-from django.db.models import Count, Q
+from django.db.models import Count, Exists, OuterRef, Q
 from drf_spectacular.utils import extend_schema_field
 from rest_framework import permissions, serializers
 from rest_framework.exceptions import PermissionDenied, ValidationError
@@ -575,6 +575,17 @@ class DatasetSerializer(serializers.ModelSerializer):
             raise ValidationError("Either do not specify set names to use the default values, or specify a non-empty list of names.")
         return set_names
 
+    def validate_unique_elements(self, unique):
+        if unique is True and self.instance and Exists(
+            DatasetElement.objects
+            .filter(set__dataset_id=OuterRef(self.instance.pk))
+            .values("element")
+            .annotate(dups=Count("element"))
+            .filter(dups__gte=2)
+        ):
+            raise ValidationError("Elements are currently contained by multiple sets.")
+        return unique
+
     def validate(self, data):
         data = super().validate(data)