Handles a bad client sending malformed JSON or non-int primary keys

send_websocket_document_updated calls document.refresh_from_db()
before building overrides, which drops the custom_fields prefetch
(and its select_related("field")) set up by the view's queryset.
DocumentMetadataOverrides.from_document() then lazily reloads field
once per custom field instance. Since from_document() can't rely on
the caller having a prefetched document, select_related explicitly at
the point of use instead.

src/documents/data_models.py

@@ -129,7 +129,7 @@ class DocumentMetadataOverrides:
         )
         overrides.custom_fields = {
             custom_field.field.id: custom_field.value
-            for custom_field in doc.custom_fields.all()
+            for custom_field in doc.custom_fields.select_related("field").all()
         }
 
         groups_with_perms = get_groups_with_perms(

src/documents/serialisers.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import logging
 import math
 import re
+from collections.abc import Iterable
 from datetime import datetime
 from datetime import timedelta
 from decimal import Decimal
@@ -876,8 +877,106 @@ def validate_documentlink_targets(user, doc_ids):
         )
 
 
+# drf-writable-nested revalidates a document's custom_fields more than once
+# per request: once as the ordinary nested list, then again per-item while
+# matching existing vs. new CustomFieldInstance rows during save() -- and
+# that second pass builds a brand new serializer (and field) instance per
+# item (see its update_or_create_reverse_relations / _get_serializer_for_field),
+# so a cache on the field instance alone only helps the first pass. It does,
+# however, explicitly pass `context=self.context` to every one of those
+# fresh serializers -- the *same* dict object the outer DocumentSerializer
+# is using, not a copy. That context dict is already request-scoped (DRF
+# builds it fresh per request via get_serializer_context()), so stashing the
+# resolved CustomField objects there -- rather than in some new global/
+# thread-local cache -- lets every later pass reuse them for free while
+# staying entirely within DRF's existing, already-request-scoped machinery.
+_CUSTOM_FIELD_CONTEXT_CACHE_KEY = "_custom_field_lookup_cache"
+
+
+class _CachingCustomFieldPrimaryKeyField(serializers.PrimaryKeyRelatedField):
+    """
+    Resolves CustomField ids with as few queries as possible: a per-instance
+    cache for repeat lookups on this exact field instance, backed by a
+    shared cache on the serializer context (see _CUSTOM_FIELD_CONTEXT_CACHE_KEY
+    above) so later, separately-instantiated fields for the same request
+    reuse what was already resolved instead of re-querying.
+    """
+
+    def __init__(self, **kwargs: Any) -> None:
+        super().__init__(**kwargs)
+        self._cache: dict[int, CustomField] = {}
+
+    def _shared_cache(self) -> dict[int, CustomField]:
+        return self.context.setdefault(_CUSTOM_FIELD_CONTEXT_CACHE_KEY, {})
+
+    @staticmethod
+    def _normalize_pk(data: Any) -> int | None:
+        """
+        Returns `data` coerced to the int a valid CustomField pk would be,
+        or None if `data` isn't a plausible pk (wrong type, unhashable,
+        non-numeric, or a bool -- DRF itself rejects bools as pks since
+        `True == 1` would otherwise silently match). None tells callers to
+        leave `data` alone and let `super().to_internal_value()` report the
+        normal validation error instead of touching the cache/queryset with
+        it directly.
+        """
+        if isinstance(data, bool):
+            return None
+        try:
+            return int(data)
+        except (TypeError, ValueError):
+            return None
+
+    def prefetch(self, ids: Iterable[Any]) -> None:
+        shared_cache = self._shared_cache()
+        candidates = {pk for i in ids if (pk := self._normalize_pk(i)) is not None}
+        missing = {
+            i for i in candidates if i not in self._cache and i not in shared_cache
+        }
+        if missing:
+            for obj in self.get_queryset().filter(pk__in=missing):
+                shared_cache[obj.pk] = obj
+        for i in candidates:
+            obj = shared_cache.get(i)
+            if obj is not None:
+                self._cache[i] = obj
+
+    def to_internal_value(self, data: Any) -> CustomField:
+        pk = self._normalize_pk(data)
+        if pk is None:
+            return super().to_internal_value(data)
+        if pk in self._cache:
+            return self._cache[pk]
+        shared_cache = self._shared_cache()
+        if pk in shared_cache:
+            obj = shared_cache[pk]
+            self._cache[pk] = obj
+            return obj
+        obj: CustomField = super().to_internal_value(data)
+        self._cache[obj.pk] = obj
+        shared_cache[obj.pk] = obj
+        return obj
+
+
+class CustomFieldInstanceListSerializer(serializers.ListSerializer):
+    def to_internal_value(self, data: Any) -> list[Any]:
+        if isinstance(data, list):
+            field_ids = []
+            for item in data:
+                if not isinstance(item, dict) or "field" not in item:
+                    continue
+                try:
+                    hash(item["field"])
+                except TypeError:
+                    continue
+                field_ids.append(item["field"])
+            if field_ids:
+                self.child.fields["field"].prefetch(field_ids)
+        return super().to_internal_value(data)
+
+
 class CustomFieldInstanceSerializer(serializers.ModelSerializer[CustomFieldInstance]):
-    field = serializers.PrimaryKeyRelatedField(queryset=CustomField.objects.all())
+    field = _CachingCustomFieldPrimaryKeyField(queryset=CustomField.objects.all())
     value = ReadWriteSerializerMethodField(allow_null=True)
 
     def create(self, validated_data):
@@ -978,6 +1077,7 @@ class CustomFieldInstanceSerializer(serializers.ModelSerializer[CustomFieldInsta
 
     class Meta:
         model = CustomFieldInstance
+        list_serializer_class = CustomFieldInstanceListSerializer
         fields = [
             "value",
             "field",

src/documents/tests/test_api_custom_fields.py

@@ -5,7 +5,9 @@ from unittest.mock import ANY
 
 from django.contrib.auth.models import Permission
 from django.contrib.auth.models import User
+from django.db import connection
 from django.test import override_settings
+from django.test.utils import CaptureQueriesContext
 from guardian.shortcuts import assign_perm
 from rest_framework import status
 from rest_framework.test import APITestCase
@@ -13,6 +15,9 @@ from rest_framework.test import APITestCase
 from documents.models import CustomField
 from documents.models import CustomFieldInstance
 from documents.models import Document
+from documents.serialisers import CustomFieldInstanceSerializer
+from documents.serialisers import DocumentSerializer
+from documents.tests.factories import DocumentFactory
 from documents.tests.utils import DirectoriesMixin
 
 
@@ -530,6 +535,136 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
         doc.refresh_from_db()
         self.assertEqual(len(doc.custom_fields.all()), 10)
 
+    def test_document_serializer_custom_fields_validation_batches_field_lookup(
+        self,
+    ) -> None:
+        """
+        GIVEN:
+            - A document is being validated with several custom field values
+              at once (as happens on every PATCH/PUT/POST)
+        WHEN:
+            - The serializer is validated
+        THEN:
+            - The referenced CustomField objects are resolved with a single
+              query, not one query per custom field
+        """
+        doc = DocumentFactory(mime_type="application/pdf")
+        custom_fields = [
+            CustomField.objects.create(
+                name=f"Test Custom Field {i}",
+                data_type=CustomField.FieldDataType.STRING,
+            )
+            for i in range(5)
+        ]
+
+        serializer = DocumentSerializer(
+            doc,
+            data={
+                "custom_fields": [
+                    {"field": custom_field.id, "value": "test value"}
+                    for custom_field in custom_fields
+                ],
+            },
+            partial=True,
+        )
+
+        with CaptureQueriesContext(connection) as ctx:
+            self.assertTrue(serializer.is_valid(), serializer.errors)
+
+        custom_field_lookups = [
+            query
+            for query in ctx.captured_queries
+            if 'FROM "documents_customfield" WHERE "documents_customfield"."id"'
+            in query["sql"]
+        ]
+        self.assertEqual(
+            len(custom_field_lookups),
+            1,
+            "Expected a single batched query to resolve the custom fields, "
+            f"got {len(custom_field_lookups)}: {custom_field_lookups}",
+        )
+
+    def test_custom_field_lookup_reuses_shared_context_cache(self) -> None:
+        """
+        GIVEN:
+            - A CustomField has already been resolved once, by a serializer
+              sharing a given `context` dict
+        WHEN:
+            - A second, separately-instantiated CustomFieldInstanceSerializer
+              validates the same field id, sharing that same context
+              (this is what drf-writable-nested does: it rebuilds a fresh
+              serializer -- and fresh field instances -- per item while
+              matching existing vs. new instances during save())
+        THEN:
+            - No additional query is issued to resolve the CustomField
+        """
+        custom_field = CustomField.objects.create(
+            name="Test Custom Field",
+            data_type=CustomField.FieldDataType.STRING,
+        )
+
+        context: dict = {}
+        first_pass = CustomFieldInstanceSerializer(
+            data={"field": custom_field.id, "value": "a"},
+            context=context,
+        )
+        self.assertTrue(first_pass.is_valid(), first_pass.errors)
+
+        second_pass = CustomFieldInstanceSerializer(
+            data={"field": custom_field.id, "value": "b"},
+            context=context,
+        )
+        with CaptureQueriesContext(connection) as ctx:
+            self.assertTrue(second_pass.is_valid(), second_pass.errors)
+
+        custom_field_lookups = [
+            query
+            for query in ctx.captured_queries
+            if 'FROM "documents_customfield" WHERE "documents_customfield"."id"'
+            in query["sql"]
+        ]
+        self.assertEqual(
+            len(custom_field_lookups),
+            0,
+            "Expected the second, separately-instantiated serializer to reuse "
+            f"the already-resolved CustomField, got: {custom_field_lookups}",
+        )
+
+    def test_custom_field_validation_rejects_malformed_field_value(self) -> None:
+        """
+        GIVEN:
+            - A document is being validated with a malformed custom_fields
+              entry whose "field" value is neither a valid CustomField id
+              nor a type DRF's own PrimaryKeyRelatedField can safely reject
+              on its own (unhashable, or a non-numeric scalar)
+        WHEN:
+            - The serializer is validated
+        THEN:
+            - A normal validation error is raised, not an unhandled
+              TypeError/ValueError escaping past DRF's validation layer
+        """
+        doc = DocumentFactory(mime_type="application/pdf")
+
+        bad_field_values = {
+            "unhashable-list": [],
+            "unhashable-dict": {},
+            "non-numeric-scalar": "abc",
+        }
+        for case_id, bad_field_value in bad_field_values.items():
+            with self.subTest(case_id):
+                serializer = DocumentSerializer(
+                    doc,
+                    data={
+                        "custom_fields": [
+                            {"field": bad_field_value, "value": "test value"},
+                        ],
+                    },
+                    partial=True,
+                )
+
+                self.assertFalse(serializer.is_valid())
+                self.assertIn("custom_fields", serializer.errors)
+
     def test_change_custom_field_instance_value(self) -> None:
         """
         GIVEN:

src/documents/tests/test_data_models.py

@@ -0,0 +1,58 @@
+from django.db import connection
+from django.test import TestCase
+from django.test.utils import CaptureQueriesContext
+
+from documents.data_models import DocumentMetadataOverrides
+from documents.models import CustomField
+from documents.models import CustomFieldInstance
+from documents.tests.factories import DocumentFactory
+from documents.tests.utils import DirectoriesMixin
+
+
+class TestDocumentMetadataOverridesFromDocument(DirectoriesMixin, TestCase):
+    def test_from_document_batches_custom_field_lookup_after_refresh_from_db(
+        self,
+    ) -> None:
+        """
+        GIVEN:
+            - A document has several custom field values
+            - The document instance has just been refreshed from the database,
+              which drops any prefetched related objects (as
+              send_websocket_document_updated does before building overrides)
+        WHEN:
+            - DocumentMetadataOverrides.from_document() reads the document's
+              custom field values
+        THEN:
+            - The referenced CustomField objects are resolved with a single
+              query, not one query per custom field
+        """
+        doc = DocumentFactory(mime_type="application/pdf")
+        for i in range(5):
+            CustomFieldInstance.objects.create(
+                document=doc,
+                field=CustomField.objects.create(
+                    name=f"Test Custom Field {i}",
+                    data_type=CustomField.FieldDataType.STRING,
+                ),
+                value_text="value",
+            )
+
+        doc.refresh_from_db()
+
+        with CaptureQueriesContext(connection) as ctx:
+            overrides = DocumentMetadataOverrides.from_document(doc)
+
+        self.assertEqual(len(overrides.custom_fields), 5)
+        unbatched_field_lookups = [
+            query
+            for query in ctx.captured_queries
+            if 'FROM "documents_customfield" WHERE "documents_customfield"."id"'
+            in query["sql"]
+        ]
+        self.assertEqual(
+            unbatched_field_lookups,
+            [],
+            "Expected CustomField data to come from the CustomFieldInstance "
+            "join, not a separate per-instance lookup, "
+            f"got: {unbatched_field_lookups}",
+        )