エラーに対処

backend/src/main/java/com/example/todoapp/dto/ToBuysDTO.java

@@ -14,7 +14,7 @@ import lombok.Data;
 @Data
 public class ToBuysDTO {
    private int tobuy_id;
-   private Integer stuff_id;
+   private Long stuff_id;
    private Long user_id;
    private int price;
    private int amount;

backend/src/main/java/com/example/todoapp/model/Stocks.java

@@ -52,7 +52,7 @@ public class Stocks {
          referencedColumnName = "stuff_id",
          nullable = false
      )
-     private int stuff_id;
+     private Long stuff_id;
 
 
     /**

backend/src/main/java/com/example/todoapp/service/ToBuysService.java

@@ -69,26 +69,25 @@ public class ToBuysService {
         User user = userRepository.findByUsername(username)
                 .orElseThrow(() -> new RuntimeException("ユーザーが見つかりません: " + username));
 
-        Stuffs stuffs;
+        Stuffs stuff;
         if (toBuyDTO.getStuff_id() == null) {
             // 新しい材料を作成
-            stuffs = new Stuffs();
-            stuffs.setStuff_name(toBuyDTO.getStuff_name());
-            stuffs.setCategory(toBuyDTO.getCategory());
-            stuffs = stuffsRepository.save(stuffs); 
+            stuff = new Stuffs();
+            stuff.setStuff_name(toBuyDTO.getStuff_name());
+            stuff.setCategory(toBuyDTO.getCategory());
+            stuff = stuffsRepository.save(stuff); 
         } else {
             // 材料情報を取得
-            int stuff_id = toBuyDTO.getStuff_id();
-            Optional<Stuffs> optionalStuffs = stuffsRepository.findById(stuff_id);
+            Optional<Stuffs> optionalStuffs = stuffsRepository.findById(toBuyDTO.getStuff_id());
             if (!optionalStuffs.isPresent()) {
                 throw new RuntimeException("材料がありません");
             }
-            stuffs = optionalStuffs.get();
+            stuff = optionalStuffs.get();
         }
 
         ToBuys toBuys = new ToBuys();
         toBuys.setUser_id(user);
-        toBuys.setStuffs(stuffs);
+        toBuys.setStuff_id(stuff.getStuff_id());
         toBuys.setAmount(toBuyDTO.getAmount());
         toBuys.setStore(toBuyDTO.getShop());
 
@@ -134,7 +133,7 @@ public class ToBuysService {
 
         ToBuys toBuys = new ToBuys();
         toBuys.setUser_id(user);
-        toBuys.setStuffs(stuffs);
+        toBuys.setStuff_id(stuffs.getStuff_id());
         toBuys.setAmount(toBuyDTO.getAmount());
         toBuys.setStore(toBuyDTO.getShop());