Add plan variety change tracking

backend/apps/plans/services.py

@@ -0,0 +1,67 @@
+from django.db import transaction
+
+from .models import Plan, PlanVarietyChange
+
+
+class _NoChange:
+    pass
+
+
+NO_CHANGE = _NoChange()
+
+
+@transaction.atomic
+def update_plan_with_variety_tracking(
+    plan: Plan,
+    *,
+    crop=NO_CHANGE,
+    variety=NO_CHANGE,
+    notes=NO_CHANGE,
+    reason: str = '',
+):
+    old_variety = plan.variety
+    updated_fields = []
+
+    if crop is not NO_CHANGE:
+        plan.crop = crop
+        updated_fields.append('crop')
+    if variety is not NO_CHANGE:
+        plan.variety = variety
+        updated_fields.append('variety')
+    if notes is not NO_CHANGE:
+        plan.notes = notes
+        updated_fields.append('notes')
+
+    if updated_fields:
+        plan.save(update_fields=updated_fields)
+
+    if variety is not NO_CHANGE and _get_variety_id(old_variety) != _get_variety_id(plan.variety):
+        handle_plan_variety_change(plan, old_variety=old_variety, new_variety=plan.variety, reason=reason)
+
+    return plan
+
+
+@transaction.atomic
+def handle_plan_variety_change(plan: Plan, *, old_variety, new_variety, reason: str = ''):
+    if _get_variety_id(old_variety) == _get_variety_id(new_variety):
+        return None
+
+    change = PlanVarietyChange.objects.create(
+        field=plan.field,
+        year=plan.year,
+        plan=plan,
+        old_variety=old_variety,
+        new_variety=new_variety,
+        reason=reason,
+    )
+    process_plan_variety_change(change)
+    return change
+
+
+def process_plan_variety_change(change: PlanVarietyChange):
+    """後続 issue で施肥計画・田植え計画への移動処理を追加する入口。"""
+    return change
+
+
+def _get_variety_id(variety):
+    return getattr(variety, 'id', None)