Using traits and constraints with grails 3.x domain objects

Question

Is it possible somehow to create a trait with fields and constraints for those fields and then create domain classes that implement that trait and pick up the fields with constraints?

I have code that essentially looks like:

trait Shared {
  String sharedField

  static constraints = {
    sharedField nullable: true
  }
}

class ImplementingClass implements Shared {
  ...
}

Saving an instance of ImplementingClass with a null sharedField is then rejected with a constraint violation.

Is it possible to do this? Is there an alternative syntax that is required to use constraints and other GORM constructs in traits implemented by domain objects?

Answer 1

I had the same problem and I looked at the source code of Grails and made some experiments.

importFrom(Shared) will not work because the Grails looks for a constraints field via clazz.getDeclaredFields() what results in an empty array for traits.

Now you have two options:

1. Make a Java/Groovy Class wich looks like your trait, but only contains the properties and constraints map

   class SharedConstraints { String sharedField

    static constraints = {
      sharedField nullable: true
    }
   

   }

Now you can use SharedConstraints with importFrom

2. Create a groovy script for the constraints. This feature is mainly used for Java domain classes but also can be used for traits.

Create a SharedConstraints.groovy in the same package:

constraints = {
    sharedField nullable: true
}

If you use IntelliJ with Grails 3.0 (maybe other versions too) the script must be placed in the resources folder. If you place the file in the src/java folder as described in the Grails documentation the script gets compiled and will not work

Also note this bug https://github.com/grails/grails-core/issues/10052