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I'm trying to create a generic database insertion method in Scala using the Slick and Play! frameworks. This involves passing in a generic Form instance and the model object that it's associated to. There are two problems I'm running into that are driving me nuts at the moment:

  1. How do I instantiate a generic type?
  2. How do I dynamically generate the parameters of that generic type from generic form binding?

Code so far

/**
  * Template method for accessing a database that abstracts out
  * Database.forDataSource(DB.getDataSource()) withSession {}
  * and actual form vals; will allow for anonymous Form declarations.
  */ 
  def dbAccessThatUsesAForm[I <: AnyRef, T <: Table[I]](
    f: Form[Product], // a Form of some generic tuple or mapping
    t: T,             // a Slick table to insert I objects into
    i: Class[I]       // the actual class that I belongs to (maybe not needed)
  )(
    request: SecuredRequest[AnyContent] // this is a SecureSocial auth. thing
  ): Boolean = {

  f.bindFromRequest((request.request.body.asFormUrlEncoded).getOrElse(Map())).fold(
    errors => {
      logger.error(t.toString + "was not bound to " + t.toString + "'s Form correctly")
      false
    },
    success => {
      t.insert(new I(someParamsThatIHaveNoIdeaWhereToStart)) // DOES NOT WORK
      true
    }
  )
}

On one

type not found: I

At this point I think I'm deeply misunderstanding something about Scala generics and considering using dependency injection as a solution. Maybe pass in a function that binds a class to this method, and call that within my method? But I already have an Injector defined in Global.scala that follows this post... Dependency injection here is based on a module... but injection here isn't based on whether I'm in production or in testing...

On two

Play Forms can take tuples for their field mappings. So I tried looking up how to describe generic tuple types . Therefore I surmised that I'd be passing in a Form[Product] (API for Product and Form) instead of Form[_], and doing something like:

(the thing in the for loop won't work because productArity isn't actually part of mapping )

for( i = 1 to f.mapping.productArity ) { // pretty sure this won't work. 
  // generate the parameters and store them to some val by calling smth like
  val store Params += success.productElem(i) 
}

As you can see, I'm quite lost as to

  • how to get the number of fields in a Form because a Form is actually composed of a Seq[Mapping]

and

  • how in the world I'd store dynamically generated parameters.

Do constructors receive parameters as a tuple? Is there a sort of Parameter object I could pass in to a generic class instantiator?

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Meredith
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  • I'm probably not going to get chance to write a full answer, but I'd suggest you look at reflection for problem 1 (take an implicit TypeTag[I] parameter in your method, and use Scala's reflection API to instantiate it). For problem 2, you might want to look at shapeless, as it has various tools for abstracting over the size of a tuple. It tends to do better for static, rather than dynamic stuff though. Also, try to ignore the fact that a form is a `Seq[Mapping]`, and look at methods like `get` and `fold` that work on tuples directly. – James_pic Jul 27 '13 at 11:16
  • Instead of `new I(someParamsThatIHaveNoIdeaWhereToStart)` where you instantiate it yourself with parameters you legitimately cannot know, why not have your caller do it for you? Much like my answer to your other question :) Just ask the caller to give you a `=> I` which you call when you need a value. They can then make the procedure constructing the `I` arbitrarily complicated and you don't have to care in the least. If you might want the new instance to know about something only you know, make it a function that takes your knowledge as arguments. – Mysterious Dan Jul 27 '13 at 13:27
  • It was suggested in [another StackOverflow-Qestion](http://stackoverflow.com/q/1305563/596532) that you could use a factory-function to create the instance. This should solve your first problem. – Peanut Oct 28 '13 at 15:14

0 Answers0