Grails Gems: Command Objects

Besides domain objects command objects are another way to get validation and data binding of parameters. But why (or when) should you use them?
First when you do not want to persist the data. Like validating parameters for a search query.
Second when you just want a subset of the parameters which has no corresponding domain object. For example for keeping malicious data away from your domain objects.
Third when you get a delta of the new data. When you just want to add to a list and do not want to check if you get a single or a multiple value for your a parameter.

Usage

Usually you put the class of the command in the same file as the controller you use them in. The command object is declared as a parameter of the action closure. You can even use multiple one:

class MyController {
  def action = { MyCommand myCommand, YourCommand yourCommand ->
    ...
  }
}

Grails automatically binds the request parameters to the commands you supply and validates them. Then you can just call command.hasErrors() to see if the validation succeeded.

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