A-2 実装内容まとめ:

バックエンド: POST /api/plans/bulk_update/ API（field_ids, year, crop, variety を受けて一括設定）
フロントエンド: チェックボックス列、全選択/個別選択、一括操作バー（作物・品種セレクタ + 確認ダイアログ）

backend/apps/plans/views.py

@@ -79,6 +79,44 @@ class PlanViewSet(viewsets.ModelViewSet):
 
         return Response({'message': f'Copied {len(new_plans)} plans from {from_year} to {to_year}'})
 
+    @action(detail=False, methods=['post'])
+    def bulk_update(self, request):
+        """複数圃場の作付け計画を一括更新"""
+        field_ids = request.data.get('field_ids', [])
+        year = request.data.get('year')
+        crop_id = request.data.get('crop')
+        variety_id = request.data.get('variety')
+
+        if not field_ids or not year or not crop_id:
+            return Response({'error': 'field_ids, year, crop are required'}, status=status.HTTP_400_BAD_REQUEST)
+
+        try:
+            crop = Crop.objects.get(id=crop_id)
+        except Crop.DoesNotExist:
+            return Response({'error': 'Crop not found'}, status=status.HTTP_400_BAD_REQUEST)
+
+        variety = None
+        if variety_id:
+            try:
+                variety = Variety.objects.get(id=variety_id)
+            except Variety.DoesNotExist:
+                pass
+
+        updated = 0
+        created = 0
+        for field_id in field_ids:
+            plan, was_created = Plan.objects.update_or_create(
+                field_id=field_id,
+                year=year,
+                defaults={'crop': crop, 'variety': variety}
+            )
+            if was_created:
+                created += 1
+            else:
+                updated += 1
+
+        return Response({'created': created, 'updated': updated, 'total': created + updated})
+
     @action(detail=False, methods=['get'])
     def get_crops_with_varieties(self, request):
         crops = Crop.objects.prefetch_related('varieties').all()