无法为用户类型构造读取实例。Scala中Doobie的类型错误
我正在尝试使用doobie、http4s和CAT从数据库返回用户记录。我已被类型系统阻止,该系统根据以下代码提供以下错误: 路由器:无法为用户类型构造读取实例。Scala中Doobie的类型错误,scala,scala-cats,cats-effect,doobie,Scala,Scala Cats,Cats Effect,Doobie,我正在尝试使用doobie、http4s和CAT从数据库返回用户记录。我已被类型系统阻止,该系统根据以下代码提供以下错误: 路由器: val httpRoutes = HttpRoutes.of[IO] { case GET -> Root / "second" / id => val intId : Integer = Integer.parseInt(id) //if i make thie ConnectionIO[Option[Unit]] i
val httpRoutes = HttpRoutes.of[IO] {
case GET -> Root / "second" / id =>
val intId : Integer = Integer.parseInt(id)
//if i make thie ConnectionIO[Option[Unit]] it compiles, but returns a cats Free object
val userOption: ConnectionIO[Option[User]] = UserModel.findById(intId, transactor.transactor)
Ok(s"userOption is instance of: ${userOption.getClass} object: ${userOption.toString}")
}.orNotFound
型号:
case class User(
id: Read[Integer],
username: Read[String],
email: Read[String],
passwordHash: Read[String], //PasswordHash[SCrypt],
isActive: Read[Boolean],
dob: Read[Date]
) {
// def verifyPassword(password: String) : VerificationStatus = SCrypt.checkpw[cats.Id](password, passwordHash)
}
object UserModel {
def findById[User: Read](id: Integer, transactor: Transactor[ConnectionIO]): ConnectionIO[Option[User]] = findBy(fr"id = ${id.toString}", transactor)
private def findBy[User: Read](by: Fragment, transactor: Transactor[ConnectionIO]): ConnectionIO[Option[User]] = {
(sql"SELECT id, username, email, password_hash, is_active, dob FROM public.user WHERE " ++ by)
.query[User]
.option
.transact(transactor)
}
}
错误:
Error:(35, 70) Cannot find or construct a Read instance for type:
core.model.User
This can happen for a few reasons, but the most common case is that a data
member somewhere within this type doesn't have a Get instance in scope. Here are
some debugging hints:
- For Option types, ensure that a Read instance is in scope for the non-Option
version.
- For types you expect to map to a single column ensure that a Get instance is
in scope.
- For case classes, HLists, and shapeless records ensure that each element
has a Read instance in scope.
- Lather, rinse, repeat, recursively until you find the problematic bit.
You can check that an instance exists for Read in the REPL or in your code:
scala> Read[Foo]
and similarly with Get:
scala> Get[Foo]
And find the missing instance and construct it as needed. Refer to Chapter 12
of the book of doobie for more information.
val userOption: ConnectionIO[Option[User]] = UserModel.findById(intId, transactor.transactor)
如果我将行更改为ConnectionIO[Option[User]到ConnectionIO[Option[Unit]],它会编译并运行,但会从cats库中返回一个免费的(…)对象,我不知道如何解析它,我不明白为什么我不能返回我的case类
另请参见findBy和findById方法上的类型声明。在添加这些类型声明之前,存在一个编译错误,表示它找到了一个用户,但需要一个Read[User]。我尝试将相同的类型声明应用于路由器中的findById调用,但它给出了上面提供的相同错误
提前感谢您的帮助,请耐心等待我的无知。我从未遇到过比我更聪明的打字系统!这里有很多东西需要解开
Read
中的User
中包装字段User
参数化函数是不必要的,因为您知道要返回的类型Read
实例,则说明您做错了什么。构建Read
实例仅在您读取的数据没有直接映射到您的类型时才有用事务处理程序
是指通过调用连接,在事务中执行操作,并处理所述操作,将连接(JDBC连接上的某些操作)转换为其他monad(例如IO
)
这样做没有多大意义,可能会导致死锁(因为在保持连接的同时,您最终会尝试调用连接)。只需在ConnectionIO
中编写数据库逻辑,然后处理整个事情
Integer
除了用于与Java的互操作之外,在Scala代码中没有使用,Doobie也没有它的Get
/Put
实例ConnectionIO[Option[User]]
,然后使用.toString
。这并不是你想要的-它只是将你构建的动作变成一个无用的字符串,而没有实际计算它。要真正获得Option[User]
,你需要计算你的动作import java.util.Date
导入cats.effect.IO
导入doobie.{connectiono,Fragment,transactior}
导入doobie.its_
导入org.http4s.HttpRoutes
导入org.http4s.dsl.io_
导入org.http4s.syntax.kleisli_
def httpRoutes(transactior:transactior[IO])=httpRoutes.of[IO]{
case GET->Root/“second”/IntVar(intId)=>
UserModel.findById(intId)
.transact(交易人)
.flatMap{userOption=>
确定(s“userOption是:${userOption.getClass}对象:${userOption.toString}的实例”)
}
}奥诺特先生
最终案例类用户(
id:Int,
用户名:String,
电子邮件:String,
passwordHash:String,
isActive:布尔值,
dob:日期
)
对象用户模型{
def findById(id:Int):ConnectionIO[Option[User]]=findBy(fr“id=${id.toString}”)
private def findBy(by:Fragment):ConnectionIO[Option[User]]=
(sql“选择id、用户名、电子邮件、密码\u散列,处于活动状态,dob FROM public.user,其中“++by”)
.query[用户]
选项
}
userOption
这里是Option[User]
非常感谢@DmitryPolienko花时间解决我的问题!非常感谢您的专业知识!