model關聯的失敗異常

u1=User.new(name:"美國")
   (0.7ms)  SELECT sqlite_version(*)
u1.save
 TRANSACTION (0.1ms)  begin transaction
  User Create (1.0ms)  INSERT INTO "users" ("name", "created_at", "updated_at") VALUES (?, ?, ?)  [["name", "美國"], ["created_at", "2022-05-29 14:44:39.616534"], ["updated_at", "2022-05-29 14:44:39.616534"]]
  TRANSACTION (1.2ms)  commit transaction
 => true
s1=Store.new(title:"costco")
u1.store = s1
  Store Load (0.3ms)  SELECT "stores".* FROM "stores" WHERE "stores"."user_id" = ? LIMIT ?  [["user_id", 1], ["LIMIT", 1]]
  TRANSACTION (0.0ms)  begin transaction
  Store Create (0.9ms)  INSERT INTO "stores" ("title", "user_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["title", "costco"], ["user_id", 1], ["created_at", "2022-05-29 14:45:20.222725"], ["updated_at", "2022-05-29 14:45:20.222725"]]
  TRANSACTION (1.1ms)  commit transaction
接下來處理Product:
p1=Product.new(name:"衛生紙")
重點來了，先把store的id塞給product:
p1.store_id = s1.id
 => 1
打p1確認一下
這時可以得知，p1已經有store_id了，所以上述問題皆沒問題。