在Slick官方文档中描述：连接后台数据库后，需要通过定义Projection，即def * 来进行具体库表列column的选择和排序。通过Projection我们可以选择库表中部分列、也可以增加一些自定义列computed column。具体来说Projection提供了数据库表列与Scala值的对应。例如def * = (column1,column2)把库表的column1和column2与(Int,String)对应，column1[Int]，column2[String]。也可以说是与定义column的类参数进行对应。从Slick源代码中我们可以找到Projection定义：
abstract class AbstractTable[T](val tableTag: Tag, val schemaName: Option[String], val tableName: String) extends Rep[T] { /** The client-side type of the table as defined by its * projection */
type TableElementType
... /** The * projection of the table used as default for queries and inserts.
* Should include all columns as a tuple, HList or custom shape and optionally
* map them to a custom entity type using the <> operator.
* The `ProvenShape` return type ensures that
* there is a `Shape` available for translating between the `Column`-based
* type in * and the client-side type without `Column` in the table's type
* parameter. */
def * : ProvenShape[T]
...
}
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