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我正在尝试使用 doobie、http4s 和猫从数据库中返回用户记录。我一直被类型系统所阻碍，它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]] 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

模型：

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 cat 库中返回一个 Free(...) 对象，我无法弄清楚如何解析，我不明白为什么我不能返回我的案例类！

另请参阅 findBy 和 findById 方法的类型声明。在我添加之前，有一个编译错误说它找到了一个用户，但需要一个读取 [用户]。我尝试将相同的类型声明应用于路由器中 findById 的调用，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。我从来没有遇到过比我更聪明的类型系统！

4

1 回答 1

3

这里有很多东西要解压...

  1. 您不需要将字段包装在Userin 中Read。
  2. 没有必要对函数进行参数化User，因为您知道要返回的类型。
  3. 大多数情况下，如果您手动处理Read实例，您就做错了。Read仅当您正在读取的数据不直接映射到您的类型时，构建实例才有用。
  4. Transactor意指通过调用连接、在事务中执行操作以及处理所述操作，从ConnectionIO（通过 JDBC 连接进行的某些操作）转换为某些其他 monad（例如）。这样做并没有多大意义，并且可能会导致死锁（因为您最终会在保持连接时尝试调用连接）。只需在 中编写您的数据库逻辑，然后再编写整个事情。IOTransactor[ConnectionIO]ConnectionIOtransact
  5. Integer除了与 Java 互操作外，它不用于 Scala 代码，而且 Doobie 没有Get/Put实例。
  6. 在你的路线中，你采取ConnectionIO[Option[User]]，并且做到.toString。这不会做你想做的事 - 它只是将你构建的操作变成一个无用的字符串，而不实际评估它。要真正得到一个，Option[User]你需要评估你的行为。

把所有这些放在一起，我们最终得到了这样一段代码：

import java.util.Date
import cats.effect.IO
import doobie.{ConnectionIO, Fragment, Transactor}
import doobie.implicits._
import org.http4s.HttpRoutes
import org.http4s.dsl.io._
import org.http4s.syntax.kleisli._

def httpRoutes(transactor: Transactor[IO]) = HttpRoutes.of[IO] {
  case GET -> Root / "second" / IntVar(intId) =>
    UserModel.findById(intId)
      .transact(transactor)
      .flatMap { userOption =>
        Ok(s"userOption is instance of: ${userOption.getClass} object: ${userOption.toString}")
      }
}.orNotFound

final case class User(
  id: Int,
  username: String,
  email: String,
  passwordHash: String,
  isActive: Boolean,
  dob: Date
)

object UserModel {
  def findById(id: Int): ConnectionIO[Option[User]] = findBy(fr"id = ${id.toString}")

  private def findBy(by: Fragment): ConnectionIO[Option[User]] =
    (sql"SELECT id, username, email, password_hash, is_active, dob FROM public.user WHERE " ++ by)
      .query[User]
      .option
}

userOption这里是Option[User]。

于 2020-05-07T12:12:52.530 回答