Grails databinding: creating instances of an abstract class

Question

When working with Grails, I like to rely on automatic databinding and scaffolding as much as possible. I have the following problem. I have a domain class Flow that have a collection of instances of the domain class Node, being the latest one an abstract class:

class Flow {
    List nodes
    static hasMany = [ nodes: Node]
    static constraints = { nodes minSize:1 }
}
abstract class Node {
    String title
    static belongsTo = [ ownerFlow: Flow]
}

There are several classes that inherit from Node. When trying to create a Flow using databinding the following integration test fails:

void "a flow can be created from a single request using the default grails data binding"() {
  when:
    def controller = new FlowController()
    controller.request.addParameters(['nodes[0].title': 'First step'])
    new Flow(controller.params).save(flush:true)
  then:
    Flow.count() > 0
  }
}

The moment I change Node from abstract to non-abstract, the test passes. It makes total sense because Grails is not capable of create the node[0] instance, since Node is an abstract class, but the questions is:

• is there a way to specify the concrete class of the node so it can be created?

Technically is perfectly possible (in fact Grails is already doing something similar for persisting and retrieving the instances by using a classname column), but I'm not sure if this case is already considered in the databinding. If not:

• what would you think is the best solution without touching the controller?

Answer 1

You need to configure a default for the purposes of binding:

List nodes = [].withDefault { new MyConcreteNode() }