Dynamically create case class

Question

I am using a csv library that takes a case class and turns it into rows for me to read.

The syntax is pretty close to File(path).asCsvReader[caseClass]. Link to library here

However the problem is that I want to generate my case class from the tables in my database. I can receive the tables in my database and the types that the columns are (Int, Long, Double, String etc) but I do not know how to dynamically create a case class with that data since I do not know the information at compile time.

It is because of this that I can't use macros either since I do not know the table data at the compile time of the macro.

So how would I go about dynamically creating this case class once I receive the table data and then passing that case class to the csv library?

How do you imagine you will manipulate those case classes in your program without knowing what they look like? Once you answer that question, you know the shape of the structure you want to use. — Alec
I will only access that variables passed into them. For example case class blah(val s:String) I will only access the s variable. — kylepotts
In this case it will probably be easier to use a different library (or different method in the same library) which provides a generic representation of the CSV records (e.g. a tuple or a shapeless HList). — devkat

vsnyc vsnyc · Accepted Answer · 2016-08-26T03:00:48

With hints from dveim in this answer, I'm adding a second solution that works both in Scala 2.10 and 2.11 and uses Scala Toolbox. The generated cases classes unfortunately are in the default package.

Generate Case Class

/**
  * Generate a case class and persist to file
  * Can't add package to the external class
  * @param file External file to write to
  * @param className Class name
  * @param header case class parameters
  */
def writeCaseClass(file: File, className: String, header: String): Unit = {
  val writer: PrintWriter = new PrintWriter(file)
  writer.println("case class " + className + "(")
  writer.println(header)
  writer.println(") {}")
  writer.println("\nscala.reflect.classTag[" + className + "].runtimeClass")
  writer.flush()
  writer.close()
}

Compile external classes using Toolbox

/**
  * Helper method to search code files in directory
  * @param dir Directory to search in
  * @return
  */
def recursiveListFiles(dir: File): Array[File] = {
  val these = dir.listFiles
  these ++ these.filter(_.isDirectory).flatMap(recursiveListFiles)
}  

/**
  * Compile scala files and keep them loaded in memory
  * @param classDir Directory storing the generated scala files
  * @throws IOException if there is problem reading the source files
  * @return Map containing Class name -> Compiled Class Reference
  */
@throws[IOException]
def compileFiles(classDir: String): Map[String, Class[_]] = {
  val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()

  val files = recursiveListFiles(new File(classDir))
    .filter(_.getName.endsWith("scala"))
  println("Loaded files: \n" + files.mkString("[", ",\n", "]"))

  files.map(f => {
    val src = Source.fromFile(f).mkString
    val clazz = tb.compile(tb.parse(src))().asInstanceOf[Class[_]]
    getClassName(f.getName) -> clazz
  }).toMap
}

Instantiate and Use compiled classes

The compiled class can be obtained from the map and used where necessary e.g.

//Test Address class
def testAddress(map: Map[String, Class[_]]) = {
  val addressClass = map("AddressData")
  val ctor = addressClass.getDeclaredConstructors()(0)
  val instance = ctor.newInstance("123 abc str", "apt 1", "Hello world", "HW", "12345")
  //println("Instantiated class: " + instance.getClass.getName)
  println(instance.toString)
}

Dynamically create case class

3 Answers

Generate Case Class

Compile external classes using Toolbox

Instantiate and Use compiled classes