scala play 2.1.1 json功能语法将数据映射到不同的格式

我正在尝试使用函数语法编写自定义json序列化程序,我似乎无法找到解决此特定问题的正确方法.我有几个joda DateTime对象,我想以特定的格式编写用于使用它的UI.有谁能告诉我哪里出错了？

这是我正在处理的案例类,没有什么特别的.

case class Banner(
  id: Int = 0,ownerId: Int = 0,licenseeId: Option[Int] = None,statusColor: Option[String] = None,content: Option[String] = None,displayStart: DateTime = new DateTime(),displayEnd: DateTime = new DateTime(),created: DateTime = new DateTime(),updated: DateTime = new DateTime()
)

这些是我对序列化程序对象的导入.

import play.api.libs.json._
import play.api.libs.functional.syntax._
import org.joda.time.DateTime
import org.joda.time.format.DateTimeFormat

首先,joda DateTime会隐式转换为long,因此宏扩展器可以很好地工作,如果这就是我想要的.

object MySerializers {
  implicit val writesBanner = Json.writes[Banner]
}

我需要的是将displayStart和displayEnd对象转换为特定的字符串格式而不是long值.这就是我试图做的事情.

object MySerializers {
  val userDateFormatter = DateTimeFormat.forPattern("MM/dd/yyyy HH:mm a")

  implicit val writesBanner = (
    (__  "id").write[Int] and
    (__  "ownerId").write[Int] and
    (__  "licenseeId").write[Int] and
    (__  "statusColor").writeNullable[String] and
    (__  "content").writeNullable[String] and
    (__  "displayStart").write[DateTime].inmap[String](dt => userDateFormatter.print(dt)) and
    (__  "displayEnd").write[DateTime].inmap[String](dt => userDateFormatter.print(dt)) and
    (__  "created").write[DateTime] and
    (__  "updated").write[DateTime]
  )(unlift(Banner.unapply))
}

但编译器对此并不满意,所以我似乎并不理解使用inmap函数的正确方法.

could not find implicit value for parameter fu: 
play.api.libs.functional.InvariantFunctor[play.api.libs.json.OWrites]
[error]     (__  "displayStart").write[DateTime].inmap[String](dt =>     
userDateFormatter.print(dt)) and
[error]                                ^

任何建议都非常感谢.

implicit val writesBanner = (
    (__  "id").write[Int] and
    (__  "ownerId").write[Int] and
    (__  "licenseeId").writeNullable[Int] and
    (__  "statusColor").writeNullable[String] and
    (__  "content").writeNullable[String] and
    (__  "displayStart").write[String].contramap[DateTime](dt => userDateFormatter.print(dt)) and
    (__  "displayEnd").write[String].contramap[DateTime](dt => userDateFormatter.print(dt)) and
    (__  "created").write[DateTime] and
    (__  "updated").write[DateTime]
  )(unlift(Banner.unapply))

implicit val readsBanner = (
    (__  "id").read[Int] and
    (__  "ownerId").read[Int] and
    (__  "licenseeId").readNullable[Int] and
    (__  "statusColor").readNullable[String] and
    (__  "content").readNullable[String] and
    (__  "displayStart").read[String].fmap[DateTime](dt => DateTime.parse(dt,userDateFormatter)) and
    (__  "displayEnd").read[String].fmap[DateTime](dt => DateTime.parse(dt,userDateFormatter)) and
    (__  "created").read[DateTime] and
    (__  "updated").read[DateTime]
  )(Banner)