Acyclic relationship validation in Grails

A common domain class use-case is for a self-referential relationship that must not be circular (a directed acyclic graph). For example, a Person class may have a parent property that is a many-to-one relationship with another Person instance. However a given Person cannot be his own parent or ancestor.

We're using just such a relationship for trees of pages that inherit certain characteristics from their ancestors. In order to validate that our users aren't setting up circular references I implemented the following constraint:Then I just needed to register my new constraint by adding ConstrainedProperty.registerNewConstraint(AcyclicConstraint.NAME, AcyclicConstraint) to grails-app/conf/spring/resources.groovy. Using the constraint is as simple as this:

class Person {

    String name
    Person parent

    static constraints = {
        parent acyclic: true
    }
}

The constraint can be mixed with others such as nullable: true. The really nice thing is that the constraint implementation doesn't reference any of my domain classes directly, meaning it can be re-used in any other domain class.

When you're using similar validation logic in multiple classes defining a constraint like this is a much better option than using a validator closure and like many things in Grails it turns out to be pretty easy to implement.