问如何解决“无法解决符号===”的错误

EN

val questionToAnswer = CustomTypesMode.oneToManyRelation(questions,answers) via ((q,a) => q.id === a.question)

val scalatraVersion = "2.4.0-RC2-2"

libraryDependencies ++= Seq(
  "org.scalatra" %% "scalatra" % scalatraVersion,
  "org.eclipse.jetty" % "jetty-webapp" % "9.3.0.M2",
  "org.scalaquery" % "scalaquery_2.9.1" % "0.10.0-M1",
  "org.squeryl" % "squeryl_2.8.0" % "0.9.5-RC2",
  "joda-time" % "joda-time" % "2.9.1",
  "postgresql" % "postgresql" % "9.1-901.jdbc4",
  "org.eclipse.jetty.orbit" % "javax.servlet" % "3.0.0.v201112011016")

object AppSchema extends Schema{
  val questions = table[Question]
  val answers = table[Answer]
  val questionToAnswer = CustomTypesMode.oneToManyRelation(questions,answers) via ((q,a) => q.id === a.question)
}

case class Question (
               id:Long,
               title:String,
               content: String,
               rank: Int,
               createdDate: DateTime,
               updatedDate: Option[DateTime]
                 ){
  lazy val answers = AppSchema.questionToAnswer.left(this)
}

case class Answer(
            id: Long,
            content: String,

            rank: Int,
            createdDate: DateTime,
            updatedDate: Option[DateTime]
              ){
  lazy val question = AppSchema.questionToAnswer.right(this)
}