Try to use a batched method of tag M2M validation

src/documents/serialisers.py

@@ -48,6 +48,7 @@ from rest_framework import serializers
 from rest_framework.exceptions import PermissionDenied
 from rest_framework.fields import SerializerMethodField
 from rest_framework.filters import OrderingFilter
+from rest_framework.relations import MANY_RELATION_KWARGS
 
 if settings.AUDIT_LOG_ENABLED:
     from auditlog.context import set_actor
@@ -687,10 +688,56 @@ class CorrespondentField(serializers.PrimaryKeyRelatedField[Correspondent]):
         return Correspondent.objects.all()
 
 
+class BatchedManyRelatedField(serializers.ManyRelatedField):
+    """
+    ManyRelatedField that validates all PKs with a single filter(pk__in=pks)
+    instead of one queryset.get() per PK, eliminating N+1 on write validation.
+    """
+
+    def to_internal_value(self, data):
+        if isinstance(data, str) or not hasattr(data, "__iter__"):
+            self.fail("not_a_list", input_type=type(data).__name__)
+        if not self.allow_empty and len(data) == 0:
+            self.fail("empty")
+        if not data:
+            return []
+
+        child = self.child_relation
+        pk_field = getattr(child, "pk_field", None)
+
+        pks = []
+        for item in data:
+            if pk_field is not None:
+                item = pk_field.to_internal_value(item)
+            if isinstance(item, bool):
+                child.fail("incorrect_type", data_type="bool")
+            pks.append(item)
+
+        try:
+            found = {obj.pk: obj for obj in child.get_queryset().filter(pk__in=pks)}
+        except (TypeError, ValueError):
+            child.fail("incorrect_type", data_type=type(pks[0]).__name__)
+
+        result = []
+        for pk in pks:
+            if pk not in found:
+                child.fail("does_not_exist", pk_value=pk)
+            result.append(found[pk])
+        return result
+
+
 class TagsField(serializers.PrimaryKeyRelatedField[Tag]):
     def get_queryset(self):
         return Tag.objects.all()
 
+    @classmethod
+    def many_init(cls, *args, **kwargs):
+        list_kwargs = {"child_relation": cls(*args, **kwargs)}
+        for key, value in kwargs.items():
+            if key in MANY_RELATION_KWARGS:
+                list_kwargs[key] = value
+        return BatchedManyRelatedField(**list_kwargs)
+
 
 class DocumentTypeField(serializers.PrimaryKeyRelatedField[DocumentType]):
     def get_queryset(self):