In Slick 3.0, how to simplify nested `db.run`? - scala

Question

I'm using Slick 3.0, and following is my codes:
def registerMember(newMember: TeamMember): Future[Long] = {
db.run(
teamProfileTable.filter(u => u.ID === newMember.ID).result.headOption
).flatMap {
case None => Future(-1)
case _ => db.run(
(teamProfileTable returning teamProfileTable.map(_.staffID)) += newMember.toTeamRecord
)
}
}
This may look ok. But when there are more layers of callback, the codes may become hard to read. I tried to simplify the codes using for-expression or andThen.. But due to the pattern matching part, I can only use flatMap to implement this..
Does anyone have ideas about how to refactor this?

Answer 1

I think a for comprehension should be okay here, you just need conditional handling of the Option in the result of the first Future. Something like this should work (note I did not compile check this):
def registerMember(newMember: TeamMember): Future[Long] = {
for{
r1Opt <- db.run(teamProfileTable.filter(u => u.ID === newMember.ID).result.headOption
r2 <- r1Opt.fold(Future.successful(-1L))(r1 => db.run((teamProfileTable returning teamProfileTable.map(_.staffID)) += newMember.toTeamRecord)
} yield r2
}
You can see on the right side of the fold that I have access to the result of the first Future if it was a Some (as r1).
I would even take this a step further and create separate methods for the steps of the for comprehension to clean things up, like so:
def registerMember(newMember: TeamMember): Future[Long] = {
def findMember =
db.run(teamProfileTable.filter(u => u.ID === newMember.ID).result.headOption
def addMember(r1Opt:Option[TeamMember]) = {
r1Opt.fold(Future.successful(-1L)){r1 =>
db.run((teamProfileTable returning teamProfileTable.map(_.staffID)) +=
newMember.toTeamRecord)
}
}
for{
r1Opt <- findMember
r2 <- addMember(r1Opt)
} yield r2
}

Answer 2

Another approach to simplify nested db.runs in Slick 3.0 when the query spans two tables could be to join the queries into a single query. Joining and Zipping. However, the OP seems to have the somewhat rarer case of nested queries on the same table so this approach may not be helpful in that particular case.
val query = slickLoginInfos join slickUserLoginInfos on
((l,ul) => l.id === ul.loginInfoId)
db.run((for { (l, ul) <- query } yield (ul)).result.headOption)