Add plan variety change tracking

backend/apps/plans/serializers.py

@@ -5,6 +5,7 @@ from apps.fields.models import Field
 from apps.materials.models import StockTransaction
 from .models import Crop, Variety, Plan
 from .models import RiceTransplantEntry, RiceTransplantPlan
+from .services import NO_CHANGE, update_plan_with_variety_tracking
 
 
 class VarietySerializer(serializers.ModelSerializer):
@@ -44,10 +45,12 @@ class PlanSerializer(serializers.ModelSerializer):
         return Plan.objects.create(**validated_data)
 
     def update(self, instance, validated_data):
-        for attr, value in validated_data.items():
-            setattr(instance, attr, value)
-        instance.save()
-        return instance
+        return update_plan_with_variety_tracking(
+            instance,
+            crop=validated_data.get('crop', NO_CHANGE),
+            variety=validated_data.get('variety', NO_CHANGE),
+            notes=validated_data.get('notes', NO_CHANGE),
+        )
 
 
 class RiceTransplantEntrySerializer(serializers.ModelSerializer):