How to mix select and delete in a Slick transaction - scala

Question

Why does it not work to combine a SELECT and a DELETE statement in a Slick query? as in:
val query = (for {
item <- SomeTable
_ <- OtherTable.filter(_.id === item.id).delete
} yield ()).transactionally
"Cannot resolve symbol 'transactionally'"
(without .transactionally, it is a Query[Nothing, Nothing, Seq], if that helps)
while the two actions work separately:
val query = (for {
item <- SomeTable
} yield ()).transactionally
,
val query = (for {
_ <- OtherTable.filter(_.id === 2).delete
} yield ()).transactionally

Answer 1

OK so this is a classic example of mixing DBIO with Query.
In your first case:
val query = (for {
item <- SomeTable // this is `Query`
_ <- OtherTable.filter(_.id === item.id).delete // this is `DBIO`
} yield ()).transactionally
Obviously for DML you can use only actions (Query is for DQL - being simply SELECT).
So first thing is - change your code to use only DBIOs. Below example is incorrect.
val query = (for {
item <- SomeTable.result // this is `DBIO` now
_ <- OtherTable.filter(_.id === item.id).delete // but this won't work !!
} yield ()).transactionally
OK, we are nearly there - the problem is that it doesn't compile. What you need to do is to be aware that now this part:
item <- SomeTable.result
returns Seq of your SomeTable case class (which among other things contains your id).
So let's take into account:
val query = (for {
items <- SomeTable.result // I changed the name to `items` to reflect it's plural nature
_ <- OtherTable.filter(_.id.inset(items.map(_.id))).delete // I needed to change it to generate `IN` query
} yield ()).transactionally