slick - query composition

query

// your "subquery"
val q1 = (for {
  (a, b) <- someTable
  .leftJoin(anotherTable).on(_.id === _.other_id)
  } yield (a.id, b.id)).groupBy(_._1).map {
    case (id, group) => (id, group.map(_._2.asColumnOf[String]))
  }

val q2 = for {
  ((a, b), c) <- aaaTable
    .leftJoin(bbbTable).on(_.id === _.other_id)
    .leftJoin(q1).on(_._1 === _._1) // refers back to q1
  } yield (a, b, c)

val result = q2.list