[rules-users] [Drools Planner] Hard constraint appears to be ignored

Hello, it appears that hard constraint I have defined is being ignored in some rare cases. Here's an example: This is what my application logs after getting best solution after solver has returned from solve(): 2012-02-23 21:28:54,595 [main] INFO Project Project1 assignment 563:[2012-02-27T13:00:00.000/2012-02-27T17:00:00.000] was assigned to John 2012-02-23 21:28:54,597 [main] INFO Project Project2 assignment 508:[2012-02-27T13:00:00.000/2012-02-27T17:00:00.000] was assigned to John (The log says that John was assigned twice in same time period to two different assignments (assignment 563 and assignment 508) Here short explanation of the relevant domain data assignment.id = 563 assignment.interval = 2012-02-27T13:00:00.000/2012-02-27T17:00:00.000 assignment.project = Project1 assignment.resource = John And this is rule that, to my opinion should've prohibited that two assignments get assigned to the same resource: rule "only one assignment within interval for the same resource" when $leftAssignment : Assignment($leftId : id, $interval : interval, resource != null, $resource : resource) $rightAssignment : Assignment(interval == $interval, resource == $resource, id != $leftId) then insertLogical(new IntConstraintOccurrence("only one assignment within interval for the same resource", ConstraintType.NEGATIVE_HARD, 1, $leftAssignment, $rightAssignment)); end The score looks like this: 2012-02-23 21:28:54,594 [main] DEBUG The hard score is: 0 and the soft score is: -532 So no hard constraints are broken according to planner. My question is not what is wrong with this particular example (unless it is obvious for you), but if you can think of any anti-pattern that would cause this behavior? What I already checked is: - this happens really sporadically (most of the time the result is CORRECT but in 1 from 5 runs I get this kind of error); - I checked, re-checked and re-re-checked all the hashCode, compareTo, solutionHashCode, solutionCompare, clone, ... methods - they appear to be correct; - moves are simple and I tried to keep them consistent with the way moves in drools-planner examples are constructed; Would be great if you could give me a hint to what direction I should look next. br Reinis