Scala Case Classes to CSV

Apr 19, 2017

For the past eight months, I have have been playing and programming in Scala for fun. One of the many things I wanted to do was to convert a collection of case classes into a CSV file. This sounds like a problem that has already been solved see “PureCSV: A type-safe and boilerplate-free CSV library for Scala”, and it has… to an extent.

In my specific use case, I wanted to parse some data from JSON into a case class and eventually convert a collection of these classes into a CSV file. The problem I was running into was that the serialization library I was using, [GSON], sets JSON null values as Java null values, but Pure CSV did not like these null values and was throwing ReferenceNullExceptions.

I ended up creating a simple implementation based on this SO answer where I assign default values to different value types. Here is what I ended up doing:

import java.io.{BufferedWriter, FileWriter}

import com.github.tototoshi.csv.CSVWriter

// ...

def writeCaseClasses(seqOfCaseClasses: Seq[AnyRef], filename: String): Unit = {
  import Implicits._
  val mapOfKeyValues = seqOfCaseClasses.map(x => x.toMap)
  val values = mapOfKeyValues.map(_.values.toSeq)
  val header = mapOfKeyValues.head.keys.toSeq
    
  val out = new BufferedWriter(new FileWriter(filename))
  val writer = new CSVWriter(out)
  
  // Prepend the header to the values, alternatively you can use only the values
  writer.writeAll(Seq(header) ++ value)
}

import java.lang.reflect.Field

import org.apache.commons.lang3.StringEscapeUtils

/**
  * Implicit class that provides a toMap method. It will turn a case class into a map of key, values.
  * This is intended for case classes only.
  */
object Implicits {
  implicit class CaseClassToMap(c: AnyRef) {

    // --- Methods ---
    def toMap: Map[String, Any] = {
      toMap(getDefaultValue)
    }

    def toMap(formatFunction: (Field, AnyRef) => Any): Map[String, Any] = {
      (Map[String, Any]() /: c.getClass.getDeclaredFields) { (map: Map[String, Any], field: Field) =>
        field.setAccessible(true)
        val fieldValue: Any = formatFunction(field, c)
        map + (field.getName -> fieldValue)
      }
    }

    def getDefaultValue(field: Field, c: AnyRef): Any = {
      if (field.get(c) == null) {

        // Set the default values to something other than null
        if (field.getType.getName == "java.lang.String") ""
        else if (field.getType.getName == "int") 0
        else if (field.getType.getName == "long") 0
        else if (field.getType.getName == "double") 0.0
        else if (field.getType.getName == "boolean") false
      } else {

        // Ensure that values are not HTML escaped
        if (field.getType.getName == "java.lang.String") StringEscapeUtils.unescapeHtml4(field.get(c).toString)
        else field.get(c)
      }
    }
  }
}

Here I’m using the com.github.tototoshi.csv.CSVWriter to write to a csv file, and the org.apache.commons.lang3.StringEscapeUtils to format values, but other than that it should be all Scala.

The intention of the toMap method is to allow you to pass a function that check the value for null and set a default one. I have provided default formatting method that should cover some cases, but you can create your own and pass it to the toMap method.