如何在Squeryl中定义am:n与其他属性的关系？

case class User(id: Int, name: String)
case class Role(id: Int, name: String)

object UserRole extends Table[UserRole]("user_role") {
  val userId = column[Int]("user_id")
  val roleId = column[Int]("role_id")

  def * = (userId, roleId) <> (UserRole.apply _ tupled, UserRole.unapply)

  val user = foreignKey("user_fk", userId, Users)(_.id)
  val role = foreignKey("role_fk", roleId, Roles)(_.id)
}

val usersWithRole = from(Users, Roles, UserRole) { (u, r, ur) =>
  where(u.id === ur.userId and r.id === ur.roleId and r.name === "admin")
  select(u)
}