Question

Suppose a Car model object (as case class), immutable and created using builder pattern. (Builder pattern by Joshua Bloch).

Its build method calls a CarValidator object in order to allow creation or not of the concerned Car. Otherwise, it throws an IllegalStateException with unexpected fields precised.

Thus, no one could create a stale or invalid Car at any time at Runtime, great!

Suppose now a web form to create a Car. Play’s controller would contain this form mapping:

val carForm = Form(               //this is a conceptual sample
    mapping(
      "brand" -> nonEmptyText,
      "description" -> nonEmptyText,
      "maxSpeed" -> number
      "complexElement" -> number.verifying(........) //redundant validation here
    )(Car.apply)(Car.unapply)
  )   

In this example, there are some basics fields, but imagine more complex fields demanding complex business validations like the complexeElement here.

I really have the feeling that I would easily break the DRY(Don’t Repeat Yourself).

Indeed, whatever the form validation would bring, this would be already provided by my Car builder’s validator, since validation on model is THE most important validation place and shouldn’t depend on anything else.

I imagine a solution with a Helper class near my Controller dealing with the same validator object that my builder uses. However, it forces me to get all validation methods public in order to be called independently at any validation step of my web form (like in the code snippet above).

What would be a good practice to keep this builder principle while avoiding breaking DRY?

Answer 1

If you want to keep the builder pattern, you should not have Form create instances. The form should just make sure the information that comes in is of the correct type. The form can not create the final Car because it does not know the rules to make a Car, the builder does.

So I would say you let the form put stuff into an intermediary object (a tuple or PossibleCar case class) and build your Car (using the builder) with that object.

There is another route available, but that means that you must create (possibly complex) structures that let’s you adapt the different types of validation. Both the builder and form can then use these validations (with the help of adapters) to create valid cars. I don’t know enough about the situation you’re in to give you advise on which route to take.