CODEWITHSUNDEEP
scaladrools
GroomedGorilla
GroomedGorilla

Reputation: 1002

Retrieving Drools fact from main scope (Scala)

I'm currently inserting an object into Drools working memory and running rules on it (creating a new object rather than updating the old one since I'm using Scala immutables...). From what I've understood, typically you would say something like update(myobject) and that would update the original variable inserted, letting you use the updated myObject in the main scope once the rules have been fired.

Since the objects I'm using (and inserting to memory) are immutable I can't simply modify them, and I'm having to create copies of them with slight tweaks.

Is there a way to return an object created within a rule's RHS? Perhaps by calling its FactHandle? Also open to other workarounds...

Alternatively, can I create a new object (newObject) and assign it the original's (myObject) FactHandle? Would that give me the access I need?

(Once again, I'm looking for a workaround to get Scala and Drools to work together.)

Upvotes: 0

Views: 568

Answers (2)

GroomedGorilla
GroomedGorilla

Reputation: 1002

After looking around and some prolonged trial and error I realised that FactHandles didn't quite work the way I expected. I was under the impression that they were an ID assigned to an object and that updating the object would mean that it kept its FactHandle (apparently not).

I went about this by creating a new (Scala) object and running update in the RHS of the rule as follows:

update(kcontext.getKieRuntime().getFactHandle(myObject), newObject)

While this worked fine within the rule-firing process, it was tedious to retrieve the object from the Main app afterwards. Calling ksession.getFactHandle(myObject) on the original object returned null after the rules had been fired and, as it turns out, the FactHandle of this object had indeed changed when checked by printing it out from within a rule.

The workaround for this was to save the original object's FactHandle before firing the rules (i.e. val objectFH = ksession.insert(myObject)) and to call .getObject using that FactHandle after the rules were fired (even though the FactHandle saved in this variable had changed).

In all honesty, I'm still unsure about why this works since the FactHandle changes on update, however I'm pretty sure this stems from my lack of understanding on how FactHandles work. If anyone would care to elaborate in the comments I'd be more than happy to add it in the answer for future reference.

(Thanks to @laune for helping point me back to my previous questions)

Upvotes: 0

Josep Prat
Josep Prat

Reputation: 503

As far as I understand you problem, you could create a copy of the object with the tweaks you need and then you retract the old object and insert the new one. Something similar to:

val newObject = myObject.copy(foo = "bar");
retract(myObject);
insert(newObject);

I hope, it helps

Upvotes: 1

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