feat: ensure errors have useful error messages

src/queryspy/listeners.py

@@ -43,7 +43,7 @@ def notify(self, model: Type[models.Model], field: str):
         self.counts[key] += 1
         count = self.counts[key]
         if count >= threshold:
-            raise NPlusOneError("BAD!")
+            raise NPlusOneError(f"N+1 detected on {model.__name__}.{field}")
 
     def reset(self):
         self.counts = defaultdict(int)

src/queryspy/patch.py

@@ -1,7 +1,7 @@
 import functools
 import importlib
 import inspect
-from typing import Any, Callable, NotRequired, TypedDict, Unpack
+from typing import Any, Callable, NotRequired, Type, TypedDict, Unpack
 
 from django.db import models
 from django.db.models.fields.related_descriptors import (
@@ -93,16 +93,33 @@ def wrapper(*args, **kwargs):
     )
 
 
+def parse_related_parts(
+    model: Type[models.Model],
+    related_name: str | None,
+    related_model: Type[models.Model],
+) -> ModelAndField:
+    field_name = related_name or f"{related_model._meta.model_name}_set"
+    return (model, field_name)
+
+
 def patch_reverse_many_to_one_descriptor():
     def parser(context: QuerysetContext):
-        assert "args" in context
-        field = context["args"][0]
-        return (field.model, field.name)
+        assert (
+            "manager_call_args" in context
+            and "rel" in context["manager_call_args"]
+        )
+        rel = context["manager_call_args"]["rel"]
+        return parse_related_parts(
+            rel.model, rel.related_name, rel.related_model
+        )
 
     def patched_create_reverse_many_to_one_manager(*args, **kwargs):
+        manager_call_args = inspect.getcallargs(
+            create_reverse_many_to_one_manager, *args, **kwargs
+        )
         manager = create_reverse_many_to_one_manager(*args, **kwargs)
         manager.get_queryset = patch_queryset_function(
-            manager.get_queryset, parser
+            manager.get_queryset, parser, manager_call_args=manager_call_args
         )
         return manager
 
@@ -117,7 +134,7 @@ def parser(context: QuerysetContext):
         assert "args" in context
         descriptor = context["args"][0]
         field = descriptor.related.field
-        return (field.model, field.name)
+        return (field.related_model, field.remote_field.name)
 
     ReverseOneToOneDescriptor.get_queryset = patch_queryset_function(
         ReverseOneToOneDescriptor.get_queryset, parser
@@ -137,7 +154,7 @@ def parser(context: QuerysetContext):
         related_model = manager.target_field.related_model
         field_name = manager.prefetch_cache_name if rel.related_name else None
 
-        return (model, field_name or f"{related_model._meta.model_name}_set")
+        return parse_related_parts(model, field_name, related_model)
 
     def patched_create_forward_many_to_many_manager(*args, **kwargs):
         manager_call_args = inspect.getcallargs(

tests/test_nplusones.py

@@ -1,3 +1,5 @@
+import re
+
 import pytest
 from djangoproject.social.models import Post, Profile, User
 from queryspy import NPlusOneError, queryspy_context
@@ -12,7 +14,9 @@ def test_detects_nplusone_in_forward_many_to_one():
     [user_1, user_2] = UserFactory.create_batch(2)
     PostFactory.create(author=user_1)
     PostFactory.create(author=user_2)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on Post.author")
+    ):
         for post in Post.objects.all():
             _ = post.author.username
 
@@ -25,7 +29,9 @@ def test_detects_nplusone_in_reverse_many_to_one():
     [user_1, user_2] = UserFactory.create_batch(2)
     PostFactory.create(author=user_1)
     PostFactory.create(author=user_2)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on User.posts")
+    ):
         for user in User.objects.all():
             _ = list(user.posts.all())
 
@@ -38,7 +44,9 @@ def test_detects_nplusone_in_forward_one_to_one():
     [user_1, user_2] = UserFactory.create_batch(2)
     ProfileFactory.create(user=user_1)
     ProfileFactory.create(user=user_2)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on Profile.user")
+    ):
         for profile in Profile.objects.all():
             _ = profile.user.username
 
@@ -51,7 +59,9 @@ def test_detects_nplusone_in_reverse_one_to_one():
     [user_1, user_2] = UserFactory.create_batch(2)
     ProfileFactory.create(user=user_1)
     ProfileFactory.create(user=user_2)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on User.profile")
+    ):
         for user in User.objects.all():
             _ = user.profile.display_name
 
@@ -64,7 +74,9 @@ def test_detects_nplusone_in_forward_many_to_many():
     [user_1, user_2] = UserFactory.create_batch(2)
     user_1.following.add(user_2)
     user_2.following.add(user_1)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on User.following")
+    ):
         for user in User.objects.all():
             _ = list(user.following.all())
 
@@ -77,7 +89,9 @@ def test_detects_nplusone_in_reverse_many_to_many():
     [user_1, user_2] = UserFactory.create_batch(2)
     user_1.following.add(user_2)
     user_2.following.add(user_1)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on User.followers")
+    ):
         for user in User.objects.all():
             _ = list(user.followers.all())
 
@@ -90,7 +104,9 @@ def test_detects_nplusone_in_reverse_many_to_many_with_no_related_name():
     [user_1, user_2] = UserFactory.create_batch(2)
     user_1.blocked.add(user_2)
     user_2.blocked.add(user_1)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on User.user_set")
+    ):
         for user in User.objects.all():
             _ = list(user.user_set.all())
 
@@ -103,7 +119,9 @@ def test_detects_nplusone_due_to_deferred_fields():
     [user_1, user_2] = UserFactory.create_batch(2)
     PostFactory.create(author=user_1)
     PostFactory.create(author=user_2)
-    with pytest.raises(NPlusOneError):
+    with pytest.raises(
+        NPlusOneError, match=re.escape("N+1 detected on User.username")
+    ):
         for post in (
             Post.objects.all().select_related("author").only("author__id")
         ):
@@ -145,7 +163,6 @@ def test_works_in_web_requests(client):
     ProfileFactory.create(user=user_2)
     with pytest.raises(NPlusOneError):
         response = client.get("/users/")
-        assert response.status_code == 500
 
     # but multiple requests work fine
     response = client.get(f"/user/{user_1.pk}/")