Custom Constraints with Grails

While Grails provides a very robust validation mechanism for domain objects, we encountered an issue where the required-ness of certain fields are dependent on the value of another field in the same object.

Enter the grails constraints plugin -- this allows you to create a custom constraint that is reusable in your application.

First install the plugin by executing the command

grails install-plugin constraints

Second, create a groovy file in the grails-app\utils folder with a name ending with 'Constraint.' This file should contain a closure named validate that returns a boolean value.

The closure has three parameters passed to it:
1. the value of the field on which the constraint was placed (val)
2. the instance of the object being validated (target)
3. a collection of validation errors (not used in example)

In addition, it implicitly has access to parameters passed to the constraint via the params property.

class CreditCardNumberRequiredConstraint {
   static name = "ccReq"

   def validate = { val, target ->       
       // find the payment method type value on the object
       def paymentMethod
       if (target.metaClass.hasMetaProperty("paymentMethod")) {
     paymentMethod = target.paymentMethod
 }
 
       // cc number required, check if the value is populated
 if (params.vals.contains(paymentMethod)) {
     return val
 }
       // payment method type does not require cc number
 else {
     return true
 }
   }
}

Third, use the new constraint in your domain class. The default name is the camel-cased name of your groovy file, minus Constraint (i.e. - creditCardNumberRequired). This example has over-ridden the default by adding the static name line.

class Payment {
   String name
   String paymentMethod
   String creditCardNumber

   static constraints = {
       name(nullable:false)
       paymentMethod(nullable:false, inList:['AMEX', 'MasterCard', 'Check'])
       creditCardNumber(ccReq:[vals:['AMEX', 'Mastercard']])
   }
}

Finally, add an entry to the properties files under grails-app\i18n. The key for the entry should be of the format:

default.invalid.% camel-coded-constraint-name %.message