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?

Answer 1

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)
}

Answer 2

If you are using Scala 2.10, you can use classes in the scala.tools.nsc.interpreter package to do this. Note that this no longer works in Scala 2.11. I have asked a new question, hopefully we'll get an answer.

In Scala 2.10, using the interpreter you can compile an external class file and load it virtually.

The steps are:

• Figure out a name for your class based on convention
• Parse your CSV header to know field names and data types
• Generate a case class with the above information and write to a file on disk
• Load the generated source files and compile them using the interpreter
• classes are now ready to use.

I have built a small demo that should help.

/**
  * Location to store temporary scala source files with generated case classes
  */
val classLocation: String = "/tmp/dynacode"

/**
  * Package name to store the case classes
  */
val dynaPackage: String = "com.example.dynacsv"

/**
  * Construct this data based on your data model e.g. see data type for Person and Address below.
  * Notice the format of header, it can be substituted directly in a case class definition.
  */
val personCsv: String = "PersonData.csv"
val personHeader: String = "title: String, firstName: String, lastName: String, age: Int, height: Int, gender: Int"

val addressCsv: String = "AddressData.csv"
val addressHeader: String = "street1: String, street2: String, city: String, state: String, zipcode: String"

/**
  * Utility method to extract class name from CSV file
  * @param filename CSV file
  * @return Class name extracted from csv file name stripping out ".ext"
  */
def getClassName(filename: String): String = filename.split("\\.")(0)

/**
  * Generate a case class and persist to file
  * @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("package " + dynaPackage)
  writer.println("case class " + className + "(")
  writer.println(header)
  writer.println(") {}")
  writer.flush()
  writer.close()
}

/**
  * Generate case class and write to file
  * @param filename CSV File name (should be named ClassName.csv)
  * @param header Case class parameters. Format is comma separated: name: DataType
  * @throws IOException if there is problem writing the file
  */
@throws[IOException]
private def generateClass(filename: String, header: String) {
  val className: String = getClassName(filename)
  val fileDir: String = classLocation + File.separator + dynaPackage.replace('.', File.separatorChar)
  new File(fileDir).mkdirs
  val classFile: String = fileDir + File.separator + className + ".scala"
  val file: File = new File(classFile)

  writeCaseClass(file, className, header)
}

/**
  * 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 Classloader that contains the compiled external classes
  */
@throws[IOException]
def compileFiles(classDir: String): AbstractFileClassLoader = {
  val files = recursiveListFiles(new File(classDir))
                  .filter(_.getName.endsWith("scala"))
  println("Loaded files: \n" + files.mkString("[", ",\n", "]"))

  val settings: GenericRunnerSettings = new GenericRunnerSettings(err => println("Interpretor error: " + err))
  settings.usejavacp.value = true
  val interpreter: IMain = new IMain(settings)
  files.foreach(f => {
    interpreter.compileSources(new BatchSourceFile(AbstractFile.getFile(f)))
  })

  interpreter.getInterpreterClassLoader()
}

//Test Address class
def testAddress(classLoader: AbstractFileClassLoader) = {
  val addressClass = classLoader.findClass(dynaPackage + "." + getClassName(addressCsv))
  val ctor = addressClass.getDeclaredConstructors()(0)
  val instance = ctor.newInstance("123 abc str", "apt 1", "Hello world", "HW", "12345")
  println("Instantiated class: " + instance.getClass.getCanonicalName)
  println(instance.toString)
}

//Test person class
def testPerson(classLoader: AbstractFileClassLoader) = {
  val personClass = classLoader.findClass(dynaPackage + "." + getClassName(personCsv))
  val ctor = personClass.getDeclaredConstructors()(0)
  val instance = ctor.newInstance("Mr", "John", "Doe", 25: java.lang.Integer, 165: java.lang.Integer, 1: java.lang.Integer)
  println("Instantiated class: " + instance.getClass.getCanonicalName)
  println(instance.toString)
}

//Test generated classes
def testClasses(classLoader: AbstractFileClassLoader) = {
  testAddress(classLoader)
  testPerson(classLoader)
}

//Main method
def main(args: Array[String]) {
  try {
    generateClass(personCsv, personHeader)
    generateClass(addressCsv, addressHeader)
    val classLoader = compileFiles(classLocation)
    testClasses(classLoader)
  }
  catch {
    case e: Exception => e.printStackTrace()
  }
}

}

Output:

Loaded files: 
[/tmp/dynacode/com/example/dynacsv/AddressData.scala,
 /tmp/dynacode/com/example/dynacsv/PersonData.scala]
Instantiated class: com.example.dynacsv.AddressData
AddressData(123 abc str,apt 1,Hello world,HW,12345)
Instantiated class: com.example.dynacsv.PersonData
PersonData(Mr,John,Doe,25,165,1)

Answer 3

So basically you need a runtime information about your case class? I guess you should use ClassTag:

import scala.reflect._

def asCsvReader[T: ClassTag]: T = {
  classTag[T].runtimeClass.getDeclaredConstructor(...).newInstance(...)
  ...
}

This will allow you to instantiate your case class at runtime.

Since you can figure out the types of your CSV columns you can provide the appropriate types into getDeclaredConstructor and newInstance methods.