CODEWITHSUNDEEP
drools
InconsistentHashing
InconsistentHashing

Reputation: 311

Return type of Drools query and combining queries

I have a situation where I need to check if there are 2 EObjects of type A. For that I have written a query like:

query checkA()
$r1 : EObject(eClass().getName() ==  "A")
$r2 : EObject(eClass().getName() ==  "A",this != $r1)
end

And in the same way if 3 EObjects of type B

query checkB()
$r1 : EObject(eClass().getName() ==  "B" , some_more_constraints)
$r2 : EObject(eClass().getName() ==  "B" , some_more_constraints,this != $r1)
$r3 : EObject(eClass().getName() ==  "B" , some_more_constraints,this != $r1,this != $r2)
end

And I need to check if either 2 EObjects of type A OR 3 EObjects of type B is there. I have a rule for that :

Rule "checking"
when
checkA()
||
checkB()
then
... do_something...

When I am running the rule, it is giving a ClassCastException exception.

It would be really helpful if you please help me understand why this is happening and answer the below two questions. a) How can I implement this? b) What is the return type of either of checkA() or checkB() queries ?

If I remove the '||' condition from the rule, then it is running fine without any error.

Many thanks in advance.

Upvotes: 0

Views: 580

Answers (1)

Esteban Aliverti
Esteban Aliverti

Reputation: 6322

There are 2 important things to understand here:

  1. Queries in Drools, when used in the when section of a rule, don't return a value.
  2. The || operator doesn't work as a usual or operator in Drools.

Queries in your rules

A query (without unbounds arguments) in Drools, when used inside a the when section of a rule, behaves similar to having the same patterns you have in it copied into the rule.

Let's take for example the following DRL:

query strings()
  $s1: String()
  String(this != $s1)
end

rule "Test"
when
  strings()
then
  //do something...
end

When inserting 2 Strings (s1 and s2) in the session, we will have 2 activations of the Test rule:

  • Activation 1: [s1, s2]
  • Activation 2: [s2, s1]

OR between patterns

When Drools finds an || between the patterns of a rule, it basically splits the rule into 2. The biggest difference between this behaviour and a regular or in any other programming language is that in the case that both parts of the or operation are true, the rule will be activated twice.

So, for example, the following rule:

rule "Test"
when
  String(this == "ABC") ||
  String(this == "DEF")
then
  //Do something
end

will create 2 different rules in runtime:

rule "Test-1"
when
  String(this == "ABC")
then
  //Do something
end

rule "Test-2"
when
  String(this == "DEF")
then
  //Do something
end

One of the problems when using || is that you have to be very careful when using bound variables in your patterns, because you don't know whether they will contain a value or not.

I guess your Exception is actually caused by how are you binding your variables in the when section of your rule

I hope that now that you know a little bit more about how Drools works when using queries and the || operator, you will be able to trace down where and why the exception is happening.

Upvotes: 1

Related Questions