fix(product): deterministic catalogue pick by ordering get() by id

ProductPostgresRepository.get took .first() with no ORDER BY, so when a
measure type has several active material rows (the live catalogue holds 74
solar_pv, 5 high_heat_retention_storage_heaters) the chosen row — hence the
cost and material_id — depended on the database's physical row order. Order by
id so a re-seed prices the same product every time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

repositories/product/product_postgres_repository.py

@@ -17,11 +17,16 @@ class ProductPostgresRepository(ProductRepository):
         self._session = session
 
     def get(self, measure_type: str) -> Product:
+        # The live catalogue holds many active rows per type; order by id so the
+        # pick is deterministic (a re-seed prices the same) rather than relying
+        # on the database's physical row order.
         row: MaterialRow | None = self._session.exec(
-            select(MaterialRow).where(
+            select(MaterialRow)
+            .where(
                 col(MaterialRow.type) == measure_type,
                 col(MaterialRow.is_active).is_(True),
             )
+            .order_by(col(MaterialRow.id))
         ).first()
         if row is None:
             raise ValueError(f"no active product for measure type {measure_type!r}")

tests/repositories/product/test_product_postgres_repository.py

@@ -42,6 +42,43 @@ def test_get_maps_active_material_to_product_with_contingency(
     assert abs(product.contingency_rate - 0.10) <= 1e-9
 
 
+def test_get_picks_the_lowest_id_when_several_active_rows_share_a_type(
+    db_engine: Engine,
+) -> None:
+    # Arrange — the live catalogue holds many active rows per type (e.g. 74
+    # solar_pv); the choice must be deterministic so a re-run prices the same.
+    with Session(db_engine) as session:
+        session.add_all(
+            [
+                MaterialRow(
+                    id=7,
+                    type="solar_pv",
+                    total_cost=99.0,
+                    cost_unit="gbp_per_unit",
+                    is_active=True,
+                    description="Solar PV (higher id)",
+                ),
+                MaterialRow(
+                    id=3,
+                    type="solar_pv",
+                    total_cost=42.0,
+                    cost_unit="gbp_per_unit",
+                    is_active=True,
+                    description="Solar PV (lowest id)",
+                ),
+            ]
+        )
+        session.commit()
+
+    # Act
+    with Session(db_engine) as session:
+        product: Product = ProductPostgresRepository(session).get("solar_pv")
+
+    # Assert — the lowest-id active row wins, deterministically.
+    assert product.id == 3
+    assert abs(product.unit_cost_per_m2 - 42.0) <= 1e-9
+
+
 def test_get_raises_when_only_an_inactive_product_exists(db_engine: Engine) -> None:
     # Arrange
     with Session(db_engine) as session: