Silently dropping a nullable database column with slick - scala

Question

I have a database for objects called Campaigns containing three fields :
Id (int, not nullable)
Version (int, not nullable)
Stuff (Text, nullable)
Let's call CampaignsRow the corresponding slick entity class
When I select line from Campaigns, I don't always need to read stuff, which contains big chunks of text.
However, I'd very much like to work in the codebase with the class CampaignsRow instead of a tuple, and so to be able to sometimes just drop the Stuff column, while retaining the original type
Basically, I'm trying to write the following function :
//Force dropping the Stuff column from the current Query
def smallCampaign(campaigns: Query[Campaigns, CampaignsRow, Seq]): Query[Campaigns, CampaignsRow, Seq] = {
val smallCampaignQuery = campaigns.map {
row => CampaignsRow(row.id, row.version , None : Option[String])
}
smallCampaignQuery /* Fails because the type is now wrong, I have a Query[(Rep[Int], Rep[Int], Rep[Option[String]), (Int, Int, Option[String], Seq] */
}
Any idea how to do this ? I suspect this has to do with Shape in slick, but I can't find a resource to start understanding this class, and the slick source code is proving too complex for me to follow.

Answer 1

You're actually already doing almost what you want in def *, the default mapping. You can use the same tools in the map method. Your two tools are mapTo and <>.
As you've found, there is the mapTo method which you can only use if your case class exactly matches the shape of the tuple, so if you wanted a special case class just for this purpose:
case class CampaignLite(id: Int, version: Int)
val smallCampaignQuery = campaigns.map {
row => (row.id, row.version).mapTo[CampaignLite]
}
As you want to reuse your existing class, you can write your own convert functions instead of using the standard tupled and unapply and pass those to <>:
object CampaignRow {
def tupleLite(t: (Int, Int)) = CampaignRow(t._1, t._2, None)
def unapplyLite(c: CampaignRow) = Some((c.id, c.version))
}
val smallCampaignQuery = campaigns.map {
row => (row.id, row.version) <> (CampaignRow.tupleLite, CampaignRow.unapplyLite)
}
This gives you the most flexibility, as you can do whatever you like in your convert functions, but it's a bit more wordy.
As row is an instance of the Campaigns table you could always define it there alongside *, if you need to use it regularly.
class Campaigns ... {
...
def * = (id, version, stuff).mapTo[CampaignRow]
def liteMapping = (id, version) <> (CampaignRow.tupleLite, CampaignRow.unapplyLite)
}
val liteCampaigns = campaigns.map(_.liteMapping)
Reference: Essential Slick 3, section 5.2.1

Answer 2

If I understand your requirement correctly, you could consider making CampaignRow a case class that models your Campaigns table class by having Campaigns extend Table[CampaignRow] and providing the bidirectional mapping for the * projection:
case class CampaignRow(id: Int, version: Int, stuff: Option[String])
class Campaigns(tag: Tag) extends Table[CampaignRow](tag, "CAMPAIGNS") {
// ...
def * = (id, version, stuff) <> (CampaignRow.tupled, CampaignRow.unapply)
}
You should then be able to do something like below:
val campaigns = TableQuery[CampaignRow]
val smallCampaignQuery = campaigns.map( _.copy(stuff = None) )
For a relevant example, here's a Slick doc.