title: date range validation groovy grails
body: <p>I am new to groovy grails so any help would be appreciated.
  I am working on a grails application which tracks employee workplace history. One of the validation requirements is that the start date and end date duration for an employee should not overlap. That means the startDate..endDate range should be unique among his employment dates for every employee.
  So my question is that how should I handle this validation ? Should it be done in the static constraint block as a custom validator or it should be done in database level?</p>