The elements type of for comprehension in ScalaQuery

I'm found something interesting when I following the Queries tutorial of ScalaQuery that I don't know why quite well.

Here is my database schema defined:

object Users extends Table[(Int, String, String)]("users") {
    def id = column[Int]("id", O NotNull)
    def first = column[String]("first", O NotNull)
    def last = column[String]("last", O NotNull)
    def * = id ~ first ~ last 
}

And here is the query I use:

object Main
{
    val database = Database.forURL("jdbc:sqlite:sample.db", driver = "org.sqlite.JDBC")
    def main(args: Array[String]) {
        database withSession {

            val query1 = for (user <- Query(Users)) yield user.id ~ user.last
            val query2 = for (user <- Users if user.id > 5) yield user.id ~ user ~ last
        }
    }
}

In this case, you could see that in both query1 and query2 use something like user.id开发者_C百科, it seems that user is type of singleton object Users which I just defined above. So it has all methods defined on that object.

But if I try to run the query directly without yield keyword, for example:

for (user <- Users if user.id > 5) {
    println ("UserID:" + user.id)
}

In this case, the compiler complain:

[error] src/main/scala/Test.scala:23: value id is not a member of (Int, String, String)
[error]    println ("UserID:" + user.id)

It seems like user in the println statement is type of Tuple3. And if I use user like ordinary tuple like the following, it will work.

for (user <- Users if user.id > 5) {
    println ("UserID:" + user._1)
}

And you could see, in the guard of for expression I still use user.id, so what is the type of user? Why I could use user.id in the guard and yield block, but I need use it as a tuple in the body of for expression?

Thanks.

---

In the first code snippet:

val query1 = for (user <- Query(Users)) yield user.id ~ user.last

user is object Users because the Query's map method is defined as def map[F](f: E => F): Query[F], the first code snippet is equals:

Query(Users).map(user => user.id ~ user.last)

so the E is Users's type and Users instance is the parameter give to f(E):F.

If you want to visit row as a Object, you need define Users as below:

import org.scalaquery.ql.basic.{BasicTable => Table}
object Users extends Table[User]("users") {
  def id = column[Int]("id", O NotNull)
  def first = column[String]("first", O NotNull)
  def last = column[String]("last", O NotNull)
  def * = id ~ first ~ last <> (User, User.unapply _)
}

case class User(id: Int, first: String, last: String)

and then

for (user <- Users if user.id > 5) {
  println ("UserID:" + user.id)  // user is a User instance
}