tests/test_template_edit_isolation.py

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+from __future__ import annotations
+
+from decimal import Decimal
+
+from quartermaster import month_service, service
+from quartermaster.models import Section
+
+
+def test_template_edit_does_not_mutate_existing_month_and_applies_to_next(db):
+    # seed the template
+    twitch = service.add_entry(
+        db, Section.subscription, "Twitch", Decimal("10.99")
+    )
+
+    # create April 2026 (snapshots the current template)
+    april = month_service.create_month(db, "2026-04")
+    april_twitch = next(
+        e for e in april.entries if e.source_entry_id == twitch.id
+    )
+    assert april_twitch.planned == Decimal("10.99")
+    assert april_twitch.origin_planned == Decimal("10.99")
+    assert april_twitch.name == "Twitch"
+    assert april_twitch.origin_name == "Twitch"
+
+    # edit the template
+    service.update_entry(
+        db, twitch.id, name="Twitch Prime", amount=Decimal("11.99")
+    )
+
+    # april is untouched
+    db.refresh(april_twitch)
+    assert april_twitch.planned == Decimal("10.99")
+    assert april_twitch.origin_planned == Decimal("10.99")
+    assert april_twitch.name == "Twitch"
+    assert april_twitch.origin_name == "Twitch"
+
+    # creating May 2026 picks up the new values
+    may = month_service.create_month(db, "2026-05")
+    may_twitch = next(e for e in may.entries if e.source_entry_id == twitch.id)
+    assert may_twitch.planned == Decimal("11.99")
+    assert may_twitch.origin_planned == Decimal("11.99")
+    assert may_twitch.name == "Twitch Prime"
+    assert may_twitch.origin_name == "Twitch Prime"