Slick update operation returns before object is flushed in the database - scala

Question

I am experiencing a scenario where when I fetch an object immediately after updating it, sometimes the result I get from the DB does not contain the most recent changes.
This has led me to think that the update thread returns before the object is actually committed in the DB. Is this expected behavior?
I would think that the update method would only return after the changes have been successfully flushed to the DB however it looks like this not guaranteed.
Below is pseudo code demonstrating what I am talking about.
def processObject = {
for {
objectId: Option[Long] <- saveObjectInDb
_ <- {
//perform other synchronous business logic and then update created object details
dao.findById(objectId.get).map { objectOption: Option[MyObject] =>
dao.update(objectOption.get.copy(processingStep = "third-step"))
}
}
mostRecentMyObject <- dao.findById(objectId.get)
} yield mostRecentMyObject
}
Below is how my update logic looks like
def update(myObject: MyObject): Future[Int] = {
db.run(table.filter(_.id === myObject.id).update(myObject))
}

Answer 1

The problem is that you are not considering the inner Future returned by the update method.
Given the signature of findById:
def findById(id: Long): Future[Option[MyObject]]
the snippet:
dao.findById(objectId.get).map { objectOption: Option[MyObject] =>
dao.update(objectOption.get.copy(processingStep = "third-step"))
}
will gave an object of type Future[Future[Int]].
You should instead flatMap instead of map over the findById future, like so:
dao.findById(objectId.get).flatMap { objectOption: Option[MyObject] =>
dao.update(objectOption.get.copy(processingStep = "third-step"))
}
this will simplify to a single future (Future[Int]), and so you can be sure retrieve the object only once inserted.
Moreover you can rewrite this as:
def processObject = {
for {
objectId: Option[Long] <- saveObjectInDb
objectOption <- dao.findById(objectId.get)
_ <- dao.update(objectOption.get.copy(processingStep = "third-step"))
mostRecentMyObject <- dao.findById(objectId.get)
} yield mostRecentMyObject
}
because, into for-comprehension, the <- is a syntactic sugar for the flatMap