実装サマリー

バックエンド（3ファイル変更）
ファイル	変更内容
views.py	OfficialKyosaiFieldViewSet、OfficialChusankanFieldViewSet（ReadOnly）、紐づけ追加/解除の4つのAPIビューを追加
urls.py	紐づけ管理用の4パス追加
serializers.py	linked_field_namesフィールドを追加（紐づけ先の圃場名を返す）
keinasystem/urls.py	/api/kyosai-fields/、/api/chusankan-fields/ をルーターに登録
新規API一覧
メソッド	エンドポイント	動作確認
GET	/api/kyosai-fields/	31件返却
GET	/api/chusankan-fields/	71件返却
POST	/api/fields/{id}/kyosai-links/	{"added":1}
DELETE	/api/fields/{id}/kyosai-links/{kyosai_id}/	204
POST	/api/fields/{id}/chusankan-links/	同上
DELETE	/api/fields/{id}/chusankan-links/{chusankan_id}/	同上
フロントエンド（3ファイル変更）
ファイル	変更内容
types/index.ts	linked_field_namesプロパティ追加
fields/[id]/page.tsx	紐づけ管理UI全面実装（+追加ボタン、x解除ボタン、検索付きモーダル、面積参考表示）
fields/page.tsx	「共済」「中山間」紐づけ件数列を追加
http://localhost:3000/fields/4 などで圃場詳細画面を開いて動作確認できます。

backend/apps/fields/serializers.py

@@ -3,18 +3,28 @@ from .models import Field, OfficialKyosaiField, OfficialChusankanField
 
 
 class OfficialKyosaiFieldSerializer(serializers.ModelSerializer):
+    linked_field_names = serializers.SerializerMethodField()
+
     class Meta:
         model = OfficialKyosaiField
-        fields = ['id', 'k_num', 's_num', 'address', 'kanji_name', 'area']
+        fields = ['id', 'k_num', 's_num', 'address', 'kanji_name', 'area', 'linked_field_names']
+
+    def get_linked_field_names(self, obj):
+        return list(obj.fields.values_list('name', flat=True))
 
 
 class OfficialChusankanFieldSerializer(serializers.ModelSerializer):
+    linked_field_names = serializers.SerializerMethodField()
+
     class Meta:
         model = OfficialChusankanField
         fields = ['id', 'c_id', 'chusankan_flag', 'oaza', 'aza', 'chiban', 'branch_num',
                   'land_type', 'area', 'planting_area', 'original_crop', 'manager', 'owner',
                   'slope', 'base_amount', 'steep_slope_addition', 'smart_agri_addition',
-                  'payment_amount']
+                  'payment_amount', 'linked_field_names']
+
+    def get_linked_field_names(self, obj):
+        return list(obj.fields.values_list('name', flat=True))
 
 
 class FieldSerializer(serializers.ModelSerializer):

backend/apps/fields/urls.py

@@ -6,8 +6,12 @@ router = DefaultRouter()
 router.register(r'', views.FieldViewSet, basename='field')
 
 urlpatterns = [
-    path('', include(router.urls)),
     path('import/kyosai/', views.import_kyosai_master, name='import_kyosai'),
     path('import/yoshida/', views.import_yoshida_fields, name='import_yoshida'),
     path('import/chusankan/', views.import_chusankan_master, name='import_chusankan'),
+    path('<int:field_id>/kyosai-links/', views.add_kyosai_links, name='add_kyosai_links'),
+    path('<int:field_id>/kyosai-links/<int:kyosai_id>/', views.remove_kyosai_link, name='remove_kyosai_link'),
+    path('<int:field_id>/chusankan-links/', views.add_chusankan_links, name='add_chusankan_links'),
+    path('<int:field_id>/chusankan-links/<int:chusankan_id>/', views.remove_chusankan_link, name='remove_chusankan_link'),
+    path('', include(router.urls)),
 ]

backend/apps/fields/views.py

@@ -1,12 +1,14 @@
 import pandas as pd
 from django.db import models as django_models
 from django.http import JsonResponse
+from django.shortcuts import get_object_or_404
 from django.views.decorators.csrf import csrf_exempt
 from django.views.decorators.http import require_http_methods
-from rest_framework import viewsets, permissions, filters
-from rest_framework.decorators import action
+from rest_framework import viewsets, permissions, filters, status
+from rest_framework.decorators import action, api_view, permission_classes as perm_classes
+from rest_framework.response import Response
 from .models import OfficialKyosaiField, OfficialChusankanField, Field
-from .serializers import FieldSerializer
+from .serializers import FieldSerializer, OfficialKyosaiFieldSerializer, OfficialChusankanFieldSerializer
 
 
 class FieldViewSet(viewsets.ModelViewSet):
@@ -21,6 +23,56 @@ class FieldViewSet(viewsets.ModelViewSet):
     ordering_fields = ['group_name', 'display_order', 'id', 'area_tan']
 
 
+class OfficialKyosaiFieldViewSet(viewsets.ReadOnlyModelViewSet):
+    queryset = OfficialKyosaiField.objects.all().order_by('k_num', 's_num')
+    serializer_class = OfficialKyosaiFieldSerializer
+    permission_classes = [permissions.IsAuthenticated]
+
+
+class OfficialChusankanFieldViewSet(viewsets.ReadOnlyModelViewSet):
+    queryset = OfficialChusankanField.objects.all().order_by('c_id')
+    serializer_class = OfficialChusankanFieldSerializer
+    permission_classes = [permissions.IsAuthenticated]
+
+
+@api_view(['POST'])
+@perm_classes([permissions.IsAuthenticated])
+def add_kyosai_links(request, field_id):
+    field = get_object_or_404(Field, pk=field_id)
+    ids = request.data.get('kyosai_field_ids', [])
+    records = OfficialKyosaiField.objects.filter(id__in=ids)
+    field.kyosai_fields.add(*records)
+    return Response({'added': len(records)})
+
+
+@api_view(['DELETE'])
+@perm_classes([permissions.IsAuthenticated])
+def remove_kyosai_link(request, field_id, kyosai_id):
+    field = get_object_or_404(Field, pk=field_id)
+    kyosai = get_object_or_404(OfficialKyosaiField, pk=kyosai_id)
+    field.kyosai_fields.remove(kyosai)
+    return Response(status=status.HTTP_204_NO_CONTENT)
+
+
+@api_view(['POST'])
+@perm_classes([permissions.IsAuthenticated])
+def add_chusankan_links(request, field_id):
+    field = get_object_or_404(Field, pk=field_id)
+    ids = request.data.get('chusankan_field_ids', [])
+    records = OfficialChusankanField.objects.filter(id__in=ids)
+    field.chusankan_fields.add(*records)
+    return Response({'added': len(records)})
+
+
+@api_view(['DELETE'])
+@perm_classes([permissions.IsAuthenticated])
+def remove_chusankan_link(request, field_id, chusankan_id):
+    field = get_object_or_404(Field, pk=field_id)
+    chusankan = get_object_or_404(OfficialChusankanField, pk=chusankan_id)
+    field.chusankan_fields.remove(chusankan)
+    return Response(status=status.HTTP_204_NO_CONTENT)
+
+
 @csrf_exempt
 @require_http_methods(["POST"])
 def import_kyosai_master(request):

backend/keinasystem/urls.py

@@ -16,11 +16,18 @@ Including another URLconf
 """
 from django.contrib import admin
 from django.urls import path, include
+from rest_framework.routers import DefaultRouter
 from rest_framework_simplejwt.views import TokenObtainPairView, TokenRefreshView
+from apps.fields.views import OfficialKyosaiFieldViewSet, OfficialChusankanFieldViewSet
+
+master_router = DefaultRouter()
+master_router.register(r'kyosai-fields', OfficialKyosaiFieldViewSet, basename='kyosai-field')
+master_router.register(r'chusankan-fields', OfficialChusankanFieldViewSet, basename='chusankan-field')
 
 urlpatterns = [
     path('admin/', admin.site.urls),
     path('api/fields/', include('apps.fields.urls')),
+    path('api/', include(master_router.urls)),
     path('api/plans/', include('apps.plans.urls')),
     path('api/reports/', include('apps.reports.urls')),
     path('api/auth/jwt/create/', TokenObtainPairView.as_view(), name='token_obtain_pair'),