Perf: avoid per-instance CustomField reload in DocumentMetadataOverrides

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/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}",
+        )