Command Objects and validation

Domain objects and their constraints are great. But what if you have relatively complicated domain objects but have a page where you want specific validation, that is not tied to the domain object.
Command Objects are a great way to handle this.
At the bottom of your controller add a command object. Then you can use this command object in your controller. (Unverified, but it seemed if the command object was at the top of the controller the hot reloading stopped working)

class MyController {
  def save = {MyCommand ->
  
    // This will validate the command object based on its constraints
    cmd.hasErrors()
  }
}

class MyCommand {
  Boolean switch
  String  value  

  static constraints = {
    switch(nullable:false)
    value(nullable:false)
  }
}

And of course the command object has all the constraints available to it. So if you wanted value to be mandatory only if switch it true, do something like this

class MyCommand {
  Boolean switch
  String  value  

  static constraints = {
    switch(nullable:false)
    value(validator: {val, obj->
      if (obj.properties['switch'])
        val ? true : ['default.null.message']
    })
  }
}