[rules-users] determining what field changed

[Greg Barton]

There is no functional difference between modify and update.  "update" is the new syntax.  See section 3.3.3.1.3 of the expert doc:

https://hudson.jboss.org/hudson/job/drools/lastSuccessfulBuild/artifact/trunk/target/docs/drools-expert/html_single/index.html#d0e1264

(scroll down a bit)

As for general suggestions to prevent rule re-firing, there are several options.  The most general is "make your conditions the negation of your action."  In other words, the conditions say "the objects are in state A, and not in state B yet" and state B is what you do to them in the action.  So, to modify your example:

rule "Rule1" 
when 
      do : DataObject(FieldA > x, fieldB != value); 
then    
  modify(do.fieldB = value)
end

So something like:

rule "Rule2" 
when 
      do : DataObject(fieldB == value, fieldC != someValue); 
then    
  modify(do.fieldC = someValue)
end

rule "Rule3" 
when 
      do : DataObject(fieldC == someValToMatchForC, do.fieldA != someNewValueForA); 
then    
  modify(do.fieldA = someNewValueForA)
end

So you're always telling the engine to fire the rule if the objects are in some state, and NOT in the state you're about to put them in.

Another general pattern is to have the objects hold some artificial state (or match them with an object that holds the artificial state) that restricts rule firing to transitions in a certain logical path.  One simple way to do this is to have a String (or preferably an Enum) value in the object that informs the rules what artificial state the object is in.

rule "Rule1" 
when 
      do : DataObject(FieldA > x, state == "StageA"); 
then    
  do.setFieldB(value);
  do.setStage("StageB");
  update(do);
end

So something like:

rule "Rule2" 
when 
      do : DataObject(fieldB == value, state == "StageB"); 
then    
  do.setFieldC(someValue)
  do.setStage("StageC");
  update(do);
end

rule "Rule3" 
when 
      do : DataObject(fieldC == someValToMatchForC, state == "StageC");
then    
  do.setFieldA(someNewValueForA);
  do.setStage("StageD");
  update(do);
end

This isn't as restrictive as you might think.

There are also the flow control mechanisms available for rule flow and agenda grouping, but they don't reach down to the object level.  However, mattering on how many conflicting objects you have in memory at once, they can be quite useful for this purpose as well. (Basically, you force the entire engine into limited processing states, instead of individual objects.)

--- On Wed, 9/16/09, Chris Richmond <crichmond at referentia.com> wrote:

> From: Chris Richmond <crichmond at referentia.com>
> Subject: Re: [rules-users] determining what field changed
> To: "'Rules Users List'" <rules-users at lists.jboss.org>
> Date: Wednesday, September 16, 2009, 3:52 PM
> In general do you have any
> suggestions for getting around multiple update
> detects being made?  In other words I have a rule to
> detect if my DataObject
> has fieldA > X and if so, then modify fieldB.  Then
> another rule that checks
> if fieldB = value and then updates field C.
> 
> So something like:
> 
> rule "identify modified DATAOBJECT" 
> when 
>       do : DataObject(FieldA > x); 
> then    
>   modify(do.fieldB = value)
> end
> 
> So something like:
> 
> rule "identify modified DATAOBJECT" 
> when 
>       do : DataObject(fieldB == value); 
> then    
>   modify(do.fieldC = someValue)
> end
> 
> 
> rule "identify modified DATAOBJECT" 
> when 
>       do : DataObject(fieldC ==
> someValToMatchForC); 
> then    
>   modify(do.fieldA = someNewValueForA)
> end
> 
> 
> 
> But of course this leads to some sort of cross product
> execution as each one
> is run all over again after another makes a change and
> ramps out of control
> 
> One question.  I have been using modify() and I notice
> you had explicit:
>   do.setN00b(false);
>   update(do);
> 
> What is the difference and will that matter to my
> scenario?
> 
> Thanks,
> 
> Chris
> 
> 
> -----Original Message-----
> From: rules-users-bounces at lists.jboss.org
> [mailto:rules-users-bounces at lists.jboss.org]
> On Behalf Of Greg Barton
> Sent: Wednesday, September 16, 2009 10:14 AM
> To: Rules Users List
> Subject: Re: [rules-users] determining what field changed
> 
> Not really.  Without modifying the objects you could
> do this outside the
> rules using a WorkingMemoryEventListener, which has
> objectInserted() and
> objectUpdated() methods.  Event if you can modify the
> DataObject to add an
> "I'm a new object" flag, and set it immediately on
> insertion, then you could
> do differentiate with two rules:
> 
> rule "identify new DATAOBJECT" 
> when 
>       do : DataObject(n00b == true); 
> then    
>   do.setN00b(false);
>   update(do);
>   System.err.println("DataObject assertion detected: "
> + do);
> end 
> 
> rule "identify modified DATAOBJECT" 
> when 
>       do : DataObject(n00b == false); 
> then    
>   System.err.println("DataObject modification
> detected: " + do);
> end 
> 
> Which would still catch a spurious modification
> notification on assert!  At
> that point the semi-risky alternative is to not call
> update() in the
> "identify new DATAOBJECT" rule action.  That way, the
> "identify modified
> DATAOBJECT" is not immediately activated, but it would be
> activated the next
> time update() is called on the object.  I say
> semi-risky because it's not
> often a good idea to put a WM object into a state that the
> rete is not aware
> of, but in this case it would do what you want.
> 
> --- On Wed, 9/16/09, Chris Richmond <crichmond at referentia.com>
> wrote:
> 
> > From: Chris Richmond <crichmond at referentia.com>
> > Subject: [rules-users] determining what field changed
> > To: "'Rules Users List'" <rules-users at lists.jboss.org>
> > Date: Wednesday, September 16, 2009, 2:41 PM
> > 
> > Is it possible
> > to determine what fields from an object
> > changed within a rule.   
> > 
> > So I am using
> > this to determine if any DataObject has
> > changed:  
> > 
> > //IDENTIFY ANY DATAOBJECT
> > CHANGES 
> > 
> > rule
> > "identify any DATAOBJECT" 
> > when 
> >       $do :
> > DataObject(); 
> > then    
> > System.err.println("ANY
> > DataObject change detected:" + $do);
> > End 
> > 
> > However this
> > also fires when any new DataObject() is
> > injected into the system and the rules are fired.  I
> > would like to be able to
> > only have a rule fire when  an object is first
> > created 
> > 
> > Vs. when it is
> > modified and is there a way to detect what
> > values of an object have changed if I don’t
> > explicitly know what fields
> > of DataObject changed ahead of time? 
> > 
> > Thanks,
> > Chris 
> > 
> > -----Inline Attachment Follows-----
> > 
> > _______________________________________________
> > rules-users mailing list
> > rules-users at lists.jboss.org
> > https://lists.jboss.org/mailman/listinfo/rules-users
> > 
> 
> 
>       
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