Encoding case classes as rows

In a previous post, we’ve seen how to encode tuples as CSV rows. While useful, actual code rarely stores business objects as tuples - encoding case classes is a much more common need than encoding tuples.

Let’s imagine that we have a list of values of type Person to encode:

case class Person(id: Int, name: String, age: Int)

val ps = List(Person(0, "Nicolas", 38), Person(1, "Kazuma", 1), Person(2, "John", 18))

If we find ourselves in the easy case - that is, we don’t mind a shapeless dependency and the expected order of cells in the output CSV matches exactly the order of fields in our case class, we can just let the compiler work out how to do that.

You’ll first need to add a dependency to the generic module in your build.sbt:

libraryDependencies += "com.nrinaudo" %% "kantan.csv-generic" % "0.7.0"

Then, with the appropriate imports:

import kantan.csv._
import kantan.csv.ops._
import kantan.csv.generic._

// File in which we'll be writing the CSV data.
val out = java.io.File.createTempFile("kantan.csv", "csv")

val writer = out.asCsvWriter[Person](rfc.withHeader("Column 1", "Column 2", "Column 3"))

We’re dealing with a list of values, which CsvWriter as a helper method for which we haven’t seen before:

writer.write(ps).close()

And, through the magic of automatic type class instance derivation, everything worked out as expected:

scala.io.Source.fromFile(out).mkString
// res1: String = """Column 1,Column 2,Column 3
// 0,Nicolas,38
// 1,Kazuma,1
// 2,John,18
// """

This was the easy case though. A more complicated one is if we need to, say, encode fields in a different order than the one they were declared in the class. For this, we need to go a little bit closer to the metal and declare our own RowEncoder.

Luckily, case classes are such a common case that kantan.csv has all sorts of helpers for them - in our specific case, we’re looking for the caseEncoder method of object RowEncoder, which simply takes a list of integers mapping fields to their index in the CSV row and the case class’ unapply method:

val personEncoder: RowEncoder[Person] = RowEncoder.caseEncoder(0, 2, 1)(Person.unapply)

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