Slick / Scala：什么是Rep [Bind]，如何将其变成值？

Question

I'm trying to figure out Slick (the Scala functional relational model). I've started to build a prototype in Slick 3.0.0 but of course... most of the documentation is either out of date or incomplete.

I've managed to get to a point where I can create a schema and return an object from the database.

The problem is, what I'm getting back is a "Rep[Bind]" and not the object I would expect to get back. I can't figure out what to do this this value. For instance, if I try something like rep.countDistinct.result, I get a crash.

Here's a quick synopsis of the code... some removed for brevity:

class UserModel(tag: Tag) extends Table[User](tag, "app_dat_user_t") {
    def id = column[Long]("n_user_id", O.PrimaryKey)
    def created = column[Long]("d_timestamp_created")

    def * = (id.?, created) <> (User.tupled, User.unapply)
}

case class User(id: Option[Long], created: Long)

val users = TableQuery[UserModel]

(users.schema).create

db.run(users += User(Option(1), 2))

println("ID is ... " + users.map(_.id)) // prints "Rep[Bind]"... huh?

val users = for (user <- users) yield user

println(users.map(_.id).toString) // Also prints "Rep[Bind]"...

I can't find a way to "unwrap" the Rep object and I can't find any clear explanation of what it is or how to use it.

Answer 1

Rep[] is a replacement to the Column[] datatype used in slick .

users.map(_.id) returns values of the Column( 'n_user_id' ) for all rows

val result : Rep[Long] = users.map(_.id)

users.map(_.id) // => select n_user_id from app_dat_user_t;

The obtained value is of type Column[Long] [ which is now Rep[Long] ]. You cannot directly print values of the above resultSet as it is not of any scala collection type

• You can first convert it to some scala collection and then print it as below :

   var idList : List[Long] = List() users.map(_.id).forEach(id => idList = idList :+ id ) 

  println(idList)** // if you need to print all ids at once

• else you can simply use :

   users.map(_.id).forEach(id => println(id) ) // print for each id 

And ,

val users = TableQuery[UserModel] // => returns Query[UserModel, UserModel#TableElementType, Seq])

val users = for (user <- users) yield user // => returns Query[UserModel, UserModel#TableElementType, Seq])

both mean the same , So you can directly use the former and remove the latter