Groovy/Grails : How to sort the list of objects by id

Default sort() is useful for Comparable object. If your class is not a Comparable, use:

def orders = publicTrainingInstance.trainingOrder.sort { it.id }

That code will sort by using passed id.

See docs: http://groovy.codehaus.org/groovy-jdk/java/util/Collection.html#sort()