Reading a row with a NULL column causes an exception in slick - mysql

Question

I have a table with a column type date. This column accepts null values,
therefore, I declared it as an Option (see field perDate below). When I
run the select query through the application code I get the following exception
slick.SlickException: Read NULL value (null) for ResultSet column
problem.This
is the Slick table definition:
import java.sql.Date
import java.time.LocalDate
class FormulaDB(tag: Tag) extends Table[Formula](tag, "formulas") {
def sk = column[Int]("sk", O.PrimaryKey, O.AutoInc)
def formula = column[Option[String]]("formula")
def notes = column[Option[String]]("notes")
def periodicity = column[Int]("periodicity")
def perDate = column[Option[LocalDate]]("per_date")(localDateColumnType)
def * =
(sk, name, descrip, formula, notes, periodicity, perDate) <>
((Formula.apply _).tupled, Formula.unapply)
implicit val localDateColumnType = MappedColumnType.base[Option[LocalDate], Date](
{
case Some(localDate) => Date.valueOf(localDate)
case None => null
}, { sqlDate =>
if (sqlDate != null) Some(sqlDate.toLocalDate) else None
}
)
}

Answer 1

Your mapped column function just needs to provide the LocalDate to Date conversion. Slick will automatically handle Option[LocalDate] if it knows how to handle LocalDate.
That means changing your localDateColumnType to be:
implicit val localDateColumnType = MappedColumnType.base[LocalDate, Date](
Date.valueOf(_), _.toLocalDate
)
Chapter 5 of Essential Slick covers some of this, as does the section on User Defined Features in the Manual.
I'm not 100% sure why you're seeing the run-time error: my guess is that the column is being treated as an Option[Option[LocalDate]] or similar, and there's a level of null in there that's being missed.
BTW, your def * can probably be:
def * = (sk, name, descrip, formula, notes, periodicity, perDate).mapTo[Formula]
...which is a little nicer to read. The mapTo was added in Slick 3 at some point.