Non Empty lists validation in PlayFramework 2.3

Play has a lot of nice features built into it. One of them is form validation. It does this by letting you define mappings for forms:

val myForm = Form(
	mapping(
		"somefield" -> someDataType
	)
)(CaseClass.apply)(CaseClass.unapply _)

There's documentation on the basic use cases, such as making sure text fields aren't empty when submitted, or that integer values are within the proper range. One thing that isn't in the provided constraint validations is the ability to check if a list is empty or not.

It's not too difficult to define your own constraints, you simply need to create a Constraint of the appropriate type which has an appropriate method for determining whether the constraint is valid or not. The code performing the verification should return a ValidationResult, (specifically Valid or Invalid from the play.api.data.validation package). In code:

def nonEmptyList[T]: Constraint[List[T]] = Constraint[List[T]]("constraint.required") { list =>
	if (list.nonEmpty) Valid else Invalid(ValidationError("error.required"))
}

Once you've defined your validation function it is easy to use it in your .verifying clauses in form mappings:

val myFormWitList = Form(
	mapping(
  		"listItems" -> list(nonEmptyText).verifying(nonEmptyList)
	)(CaseClassFormData.apply)(CaseClassFormData.unapply _)
)

The above would give us a form validating that the form we submitted looked something like this:

<form>
	<input type="text" name="listItems[]" />
	<input type="text" name="listItems[]" />
	<input type="text" name="listItems[]" />
	<!-- etc... -->
</form>

Note that you do need the name for a repeated form element to have the [] at the end or Play won't recognize or bind it. Putting these all together we have:

package validations.forms

import play.api.data.Form
import play.api.data.Forms._
import play.api.data.format.Formats._
import play.api.data.validation.{Constraint,ValidationError,Valid,Invalid}


case class MyFormData(paths: List[String])

object ExampleForm {

  def nonEmptyList[T]: Constraint[List[T]] = Constraint[List[T]]("constraint.required") { o =>
    if (o.nonEmpty) Valid else Invalid(ValidationError("error.required"))
  }

  val exampleForm = Form(
    mapping(
      "items" -> list(nonEmptyText).verifying(nonEmptyList)
    )(MyFormData.apply)(MyFormData.unapply _)
  )
}

Pretty simple right? We can easily write custom constraints to support any logic our forms might need, verifying Enumerated values, making sure that we've actually submitted multiple elements to an endpoint, checking that the username matches some custom validation. You name it, you can validate it.