Limit autocomplete results view to 10

Prevent performance issues when there are many tags available.

wagtail/admin/tests/tests.py

@@ -374,6 +374,24 @@ class TestTagsAutocomplete(WagtailTestUtils, TestCase):
 
         self.assertEqual(data, [])
 
+    def test_tags_autocomplete_limit(self):
+        tags = [Tag(name=f"Tag {i}", slug=f"tag-{i}") for i in range(15)]
+        Tag.objects.bulk_create(tags)
+
+        # Send a request to the autocomplete endpoint with a broad search term
+        response = self.client.get(
+            reverse("wagtailadmin_tag_autocomplete"), {"term": "Tag"}
+        )
+
+        # Confirm the response is successful
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response["Content-Type"], "application/json")
+
+        data = json.loads(response.content.decode("utf-8"))
+
+        # The results should be limited to avoid performance issues (#12415)
+        self.assertEqual(len(data), 10)
+
 
 class TestMenuItem(WagtailTestUtils, TestCase):
     def setUp(self):

wagtail/admin/views/tags.py

@@ -19,7 +19,7 @@ def autocomplete(request, app_name=None, model_name=None):
 
     term = request.GET.get("term", None)
     if term:
-        tags = tag_model.objects.filter(name__istartswith=term).order_by("name")
+        tags = tag_model.objects.filter(name__istartswith=term).order_by("name")[:10]
     else:
         tags = tag_model.objects.none()