Aggregations with "Group by"

Slick also provides a groupBy method that behaves like the groupBy method of native Scala collections. Let's get a list of candidates with all the donations for each candidate:

scala> val grouped = Tables.transactions.groupBy { _.candidate }
grouped: scala.slick.lifted.Query[(scala.slick.lifted.Column[...

scala> val aggregated = grouped.map {
  case (candidate, group) =>
    (candidate -> group.map { _.amount }.sum)
}
aggregated: scala.slick.lifted.Query[(scala.slick.lifted.Column[...

scala> val groupedDonations = db.withSession { 
  implicit session => aggregated.list 
}
groupedDonations: List[(String, Option[Long])] = List((Bachmann, Michele,Some(7439272)),...

Let's break this down. The first statement, transactions.groupBy { _.candidate }, specifies the key by which to group. You can think of this as building an intermediate list of (String, List[Transaction]) tuples mapping the group key to a list of all the table rows that satisfy this...