Isolating side effects for the store call

package nick

import java.time.Instant
import java.util.concurrent.atomic.AtomicInteger

import scala.util.{Failure, Random, Success, Try}

case class Dto(name: String)

case class Entity(
  var id: Int,
  var name: String,
  var createdAt: Instant
)

// like true Dao sometimes fails
class Dao {
  val random = new Random()
  var currentId = new AtomicInteger(0)

  def store(e: Entity): Unit = {
    if (random.nextInt(10) < 1) {
      throw new Exception("Database error")
    } else {
      e.id = currentId.incrementAndGet()
    }
  }
}

class MyService {
  val dao = new Dao()

  // this method requires to mock dao to be tested
  def createMetadataBefore(dto: Dto): Dto = {
    val e = convertD2E(dto)
    e.createdAt = Instant.now()
    dao.store(e)
    convertE2D(e)
  }

  // split the effectful method into 2 pure functions
  // one is before store call
  def createMetadata1(dto: Dto, storeFun: Entity ⇒ Try[Entity]): Try[Entity] = {
    val e = convertD2E(dto)
    e.createdAt = Instant.now()
    storeFun(e)
  }

  // one is after store call
  def createMetadata2(e: Entity): Dto = {
    convertE2D(e)
  }

  def createMetadataAfter(dto: Dto): Dto = {
    val storeFun: Entity ⇒ Try[Entity] = e ⇒ {
      try {
        val e1 = e.copy() // to not spoil the argument
        dao.store(e1)
        Success(e1)
      } catch {
        case x: Exception ⇒
          Failure(x)
      }
    }
    val result = createMetadata1(dto, storeFun)
    result match {
      case Success(e) ⇒ createMetadata2(e)
      case Failure(ex) ⇒ throw ex
    }
  }

  def convertE2D(e: Entity): Dto = Dto(e.name)
  def convertD2E(d: Dto): Entity = Entity(0, d.name, Instant.MIN)
}

object MyServiceTest extends App {
  val service = new MyService

  def test(): Unit = {
    val now = Instant.now()
    val storingFun: Entity ⇒ Try[Entity] =
      e ⇒ Success(e.copy(id = 1, createdAt = now))
    val dto = Dto("Hello!")
    val r1 = service.createMetadata1(dto, storingFun)
    assert(r1 == Success(Entity(1, "Hello!", now)))
    val r2 = service.createMetadata2(r1.get)
    assert(r2 == Dto("Hello!"))
  }

  test()
}