+ def validate_state_change
+ ok = true
+ if self.state_changed?
+ ok = case self.state_was
+ when nil
+ # state isn't set yet
+ true
+ when Queued
+ # Permit going from queued to any state
+ true
+ when Running
+ # From running, may only transition to a finished state
+ [Complete, Failed, Cancelled].include? self.state
+ when Complete, Failed, Cancelled
+ # Once in a finished state, don't permit any more state changes
+ false
+ else
+ # Any other state transition is also invalid
+ false
+ end
+ if not ok
+ errors.add :state, "invalid change from #{self.state_was} to #{self.state}"
+ end
+ end
+ ok
+ end