fix(views,tags): a rename onto a taken name names the taken one, not the row's old name

The rollback after the conflict puts the old name back on the row, so the refusal read
"you already have a view named 'hh'" when the taken name was 'gaming'.

backend/app/routes/saved_views.py

@@ -128,7 +128,8 @@ def update_view(
         db.commit()
     except IntegrityError:
         db.rollback()
-        raise refused(409, "view_name_taken", name=view.name) from None
+        # The name asked for: the rollback has put the view's old name back on `view`.
+        raise refused(409, "view_name_taken", name=name) from None
     return _serialize(view)
 
 

backend/app/routes/tags.py

@@ -107,7 +107,8 @@ def update_tag(
         db.commit()
     except IntegrityError:
         db.rollback()
-        raise refused(409, "tag_name_taken", name=tag.name) from None
+        # The name asked for: the rollback has put the tag's old name back on `tag`.
+        raise refused(409, "tag_name_taken", name=name) from None
     return {
         "ok": True,
         "id": tag.id,

backend/tests/test_name_taken_detail.py

@@ -0,0 +1,46 @@
+"""A rename onto a name already taken says WHICH name is taken: the one asked for, not the row's
+old name (the rollback that follows the conflict puts the old name back on the row). DB lane.
+"""
+
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import User
+
+
+def _client(db, monkeypatch, override_dep):
+    u = User(email="taken@example.com")
+    db.add(u)
+    db.commit()
+    monkeypatch.setattr(
+        state, "_configured_cache", True
+    )  # the request gate 503s an unconfigured app
+    override_dep(get_db, db.info["request"])
+    override_dep(current_user, lambda: u)
+    return TestClient(app)
+
+
+def test_renaming_a_view_onto_a_taken_name_names_the_taken_one(db, monkeypatch, override_dep):
+    c = _client(db, monkeypatch, override_dep)
+    gaming = c.post("/api/saved-views", json={"name": "gaming", "filters": {}, "source": "feed"})
+    assert gaming.status_code == 200
+    hh = c.post("/api/saved-views", json={"name": "hh", "filters": {}, "source": "feed"}).json()
+
+    r = c.patch(f"/api/saved-views/{hh['id']}", json={"name": "gaming"})
+
+    assert r.status_code == 409
+    assert r.json()["detail"] == {"code": "refused", "reason": "view_name_taken", "name": "gaming"}
+
+
+def test_renaming_a_tag_onto_a_taken_name_names_the_taken_one(db, monkeypatch, override_dep):
+    c = _client(db, monkeypatch, override_dep)
+    assert c.post("/api/tags", json={"name": "music"}).status_code == 200
+    news = c.post("/api/tags", json={"name": "news"}).json()
+
+    r = c.patch(f"/api/tags/{news['id']}", json={"name": "music"})
+
+    assert r.status_code == 409
+    assert r.json()["detail"] == {"code": "refused", "reason": "tag_name_taken", "name": "music"}