[WIP] Fix if (liftQuery(set).isEmpty)

[mdedetrich]

Fixes #1557

Progress

• Produce a minimal failing test
• Come up with a solution

Problem

This is a PR aiming to fix liftQueryScalar which is currently broken.

So the core issue we have is that is on expansion of reified liftings. Reifying a traversable liftings is done as so

private def expandLiftings(statement: Statement, emptySetContainsToken: Token => Token) = {
    Statement {
      statement.tokens.foldLeft(List.empty[Token]) {
        case (tokens, SetContainsToken(a, op, ScalarLiftToken(lift: ScalarQueryLift))) =>
          lift.value.asInstanceOf[Traversable[Any]].toList match {
            case Nil => tokens :+ emptySetContainsToken(a)
            case values =>
              val liftings = values.map(v => ScalarLiftToken(ScalarValueLift(lift.name, v, lift.encoder)))
              val separators = List.fill(liftings.size - 1)(StringToken(", "))
              (tokens :+ stmt"$a $op (") ++ Interleave(liftings, separators) :+ StringToken(")")
          }
        case (tokens, token) =>
          tokens :+ token
      }
    }
  }

More specifically val liftings = values.map(v => ScalarLiftToken(ScalarValueLift(lift.name, v, lift.encoder))) is the part that creates a List of ScalarLiftToken. Here is the current code expanding this reification

  val prepare =
    (row: context.PrepareRow) => {
      val (_, values, prepare) = liftings.foldLeft((0, List.empty[Any], row)) {
        case ((idx, values, row), lift) =>
          val encoder = lift.encoder.asInstanceOf[context.Encoder[Any]]
          val newRow = encoder(idx, lift.value, row)
          (idx + 1, lift.value :: values, newRow)
      }
      (values, prepare)
    }

The problem here occurs when you have a custom encoder for something traversable. The ClassCastException happens at encoder(idx, lift.value, row). We happen to be applying the encoder to the collection rather than every element inside the collection.

Solution

Unfortunately the solution isn't going to be nice. Either its going to be hacky but will work without any major changes, but in order to create a clean solution some major changes need to be done.

To clarify, the issue is we have no way to distinguish whether an Encoder should be used on the collection or an every element inside the collection when doing expansion. When doing the expansion we just have some generic Encoder. If our Encoder happens to be a custom encoder than we typically do want to apply it on every element (linked issue describes this in more detail) however if its for already supported types (i.e. lets say a List of ints) then we apply the encoder to the entire list. To compound the problem further, expansion is shared amongst all modules so it needs to be completely generic (so currently distinguishing between jdbc/postgres-async/cassandra etc is impossible at this point)

From what I can see, the only way to solve without major changes is to catch a ClassCastException on the assumption we are hitting this case (custom encoder that needs to be applied on every element in the collection) and then if there is a problem here then we have to rethrow this same ClassCastException

Checklist

• Unit test all changes
• Update README.md if applicable
• Add [WIP] to the pull request title if it's work in progress
• Squash commits that aren't meaningful changes
• Run sbt scalariformFormat test:scalariformFormat to make sure that the source files are formatted

@getquill/maintainers

[mdedetrich]

So it turns out that the actual issue has nothing to do with liftQueryScalar but rather that the following query is broken

  def `Ex 8 and 9 contains2`(set: Set[Int]) =
    quote {
      query[Person].filter { p =>
        if (liftQuery(set).isEmpty)
          true
        else
          liftQuery(set).contains(p.age)
      }
    }

Specifically liftQuery(set).isEmpty

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