scala - 光滑的左外连接获取整个连接行作为选项

Question

我的加入看起来像这样:

def byIdWithImage = for {
    userId <- Parameters[Long]
    (user, image) <- Users leftJoin RemoteImages on (_.imageId === _.id) if user.id === userId
} yield (user, image)

但是当 user.imageId 为 null 时，slick 在运行时失败

[SlickException: Read NULL value for column RemoteImage.url]

将产量更改为

} yield (user, image.?)

给了我一个编译时异常，它只适用于单个列

could not find implicit value for evidence parameter of type scala.slick.lifted.TypeMapper[image.type]

会有不同的方式来完成我在这里尝试做的事情吗？ (在单个查询中)

Answer 1

使用下面的代码，你可以把它写成:产量(用户，图像。也许)

case class RemoteImage(id: Long, url: URL)

class RemoteImages extends Table[RemoteImage]("RemoteImage") {
    def id = column[Long]("id", O.PrimaryKey, O.AutoInc)
    def url = column[URL]("url", O.NotNull)
    def * = id.? ~ url <> (RemoteImage.apply _, RemoteImage.unapply _)

    def maybe = id.? ~ url.? <> (applyMaybe,unapplyBlank)

    val unapplyBlank = (c:Option[RemoteImage])=>None        

    val applyMaybe = (t: (Option[Long],Option[URL])) => t match {
        case (Some(id),Some(url)) => Some(RemoteImage(Some(id),url))
        case _ => None
    } 
}