class Afsc 
{
  String id 
  String code 
  Afsc parent  

  def afscService 
  def sessionFactory
  ...
  static constraints = {
    ...
    parent (nullable: true, validator: { val, obj ->
      if (obj.id && (val?.id == obj.id)
          || obj.afscService.getChild(obj, val.code)) {
        return 'AFSC.cannot.be.cyclic'
      }
    })
  }
...
}

  def getChild(parent, targetCode) {
    def children = Afsc.findAllByParent(parent)
    def child = children.find {
      it.code == targetCode
    }
    if (!child) {
      children.each {
        child = child ?: getChild(it,targetCode)
      }
    }
    return child
  }

When I tried this, I started getting stack overflow exceptions.  Long story short, it turns out that using  findAllByParent() causes hibernate to flush changes before executing the query.  Well, guess what happens when changes are flushed?  That's right - the validator gets called.  Which calls getChild().  Which calls findAllByParent().  Which causes a flush.  Which gives us infinite recursion and a stack overflow.

Once I figured out what was going on, it wasnt too hard to find the solution - tell grails/hibernate not to automatically flush changes:

    parent (nullable: true, validator: { val, obj ->
      if (obj.id && val) {
        if (val.id == obj.id) {
          return 'AFSC.cannot.be.cyclic'
        }
        def originalFlushMode = obj.sessionFactory.currentSession.flushMode
        obj.sessionFactory.currentSession.flushMode = org.hibernate.FlushMode.MANUAL
        try {
          if (obj.afscService.getChild(obj, val.code)) {
            return 'AFSC.cannot.be.cyclic'
          }
        }
        catch(Exception e) {
          return e.getMessage()
        }
        finally {
          obj.sessionFactory.currentSession.flushMode = originalFlushMode
        }
       
      }
    })