How do you flatten a Sequence of Sequence in DBIOaction SLICK? - scala

Question

Hey guys i am new in slick, how can I flatten this sequence of sequence? so that can return the commented code
def insertIfNotExists(mapCountryStates: Map[String, Iterable[StateUtil]]): Future[Seq[Seq[StateTable]]] /*: Future[Seq[StateTable]]*/ = {
val interaction = DBIO.sequence(mapCountryStates.toSeq.map { case (alpha2Country, statesUtil) =>
val codes = statesUtil.map(_.alpha3Code)
for {
countryId <- Countries.filter(_.alpha2Code === alpha2Country).map(_.id).result.head
existing <- States.filter(s => (s.alpha3Code inSet codes) && s.countryId === countryId).result
stateTables = statesUtil.map(x => StateTable(0L, x.name, x.alpha3Code, countryId))
statesInserted <- StatesInsertQuery ++= stateTables.filter(s => !existing.exists(x => x.alpha3Code == s.alpha3Code && x.countryId == s.countryId))
} yield existing ++ statesInserted
})
db.run(interaction.transactionally)
}
if I write it here:
val interaction = DBIO.sequence(...).flatten
or here:
db.run(interaction.flatten.transactionally)
[error] Cannot prove that Seq[Seq[StateRepository.this.StateTableMapping#TableElementType]] <:< slick.dbio.DBIOAction[R2,S2,E2].
but when the application runs, because the IDE does not detect it as an error:
I update my definition with DBIO.fold:

Answer 1

It looks like you might be after DBIO.fold. This provides a way to take a number of actions and reduce them down to a single value. In this case, your single value is a Seq[StateTable] from a Seq[Seq[StateTable]].
A sketch of how this could look might be...
def insertIfNotExists(...): DBIO[Seq[StateTable]] = {
val interaction: Seq[DBIO[Seq[StateTable]]] = ...
val startingPoint: Seq[StateTable] = Seq.empty
DBIO.fold(interaction, startingPoint) {
(total, list) => total ++ list
}
}
It looks like the types will line up using fold. Hope it's of some use in your case.
There's some more information about fold in Chapter 4 of Essential Slick.

Answer 2

A viable solution should be to flatten the sequence once the Future has been completed:
def insertIfNotExists(mapCountryStates: Map[String, Iterable[StateUtil]]): Future[Seq[StateTable]] = {
val interaction = DBIO.sequence(mapCountryStates.toSeq.map { case (alpha2Country, statesUtil) =>
val codes = statesUtil.map(_.alpha3Code)
for {
countryId <- Countries.filter(_.alpha2Code === alpha2Country).map(_.id).result.head
existing <- States.filter(s => (s.alpha3Code inSet codes) && s.countryId === countryId).result
stateTables = statesUtil.map(x => StateTable(0L, x.name, x.alpha3Code, countryId))
statesInserted <- StatesInsertQuery ++= stateTables.filter(s => !existing.exists(x => x.alpha3Code == s.alpha3Code && x.countryId == s.countryId))
} yield existing ++ statesInserted
})
db.run(interaction.transactionally).map(_.flatten)
}