Scala slick query where in list

Question

I am attempting to learn to use Slick to query MySQL. I have the following type of query working to get a single Visit object:

Q.query[(Int,Int), Visit](\"\"\"
          


        

Answer 1

It doesn't work because the StaticQuery object (Q) expects to implicitly set the parameters in the query string, using the type parameters of the query method to create a sort of setter object (of type scala.slick.jdbc.SetParameter[T]).
The role of SetParameter[T] is to set a query parameter to a value of type T, where the required types are taken from the query[...] type parameters.

From what I see there's no such object defined for T = List[A] for a generic A, and it seems a sensible choice, since you can't actually write a sql query with a dynamic list of parameters for the IN (?, ?, ?,...) clause

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I did an experiment by providing such an implicit value through the following code

import scala.slick.jdbc.{SetParameter, StaticQuery => Q}

def seqParam[A](implicit pconv: SetParameter[A]): SetParameter[Seq[A]] = SetParameter {  
    case (seq, pp) =>
        for (a <- seq) {
            pconv.apply(a, pp)
        }
}

implicit val listSP: SetParameter[List[String]] = seqParam[String]

with this in scope, you should be able to execute your code

val locationCodes = List("loc1","loc2","loc3"...)
Q.query[(Int,Int,List[String]), Visit]("""
    select * from visit where vistor = ? and location_code in (?,?,?...)
""").list(visitorId,locationCodes)

But you must always manually guarantee that the locationCodes size is the same as the number of ? in your IN clause

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In the end I believe that a cleaner workaround could be created using macros, to generalize on the sequence type. But I'm not sure it would be a wise choice for the framework, given the aforementioned issues with the dynamic nature of the sequence size.