Pattern could not parse constrains with '||' (OR expression) into MVELConstraint

[Leo Lin]

Hello,

I'm using Drools 8.33.0.Final. I have code to parse RuleImpl and extract all Fields those are being used in the facts.

I found that Pattern and MVELConstraint have pretty different behaviors between '&&' expression and '||' expression.

For example, in the rule:

import com.MockObjectWithProjections
rule "test"
when
  obj : MockObjectWithProjections(value1 == 1 || value2 == 2)
  then
  end

When I'm looking at kieBase.getKiePackages(), the obj is represented as a Pattern and it has only one MVELConstraint

* MVELConstraint (value1 == 1|| value2 == 2), constrainType = UNKNOWEN

However, if it is obj : MockObjectWithProjections(value1 == 1 && value2 == 2),

The pattern has 2 constraints:

* MVELConstraint (value1 == 1), constraintType = EQUAL

* MVELConstraint (value2 == 2), constraintType = EQUAL

I found the root cause is that In Pattern build() function https://github.com/kiegroup/drools/blob/main/drools-compiler/src/main/java/org/drools/compiler/rule/builder/PatternBuilder.java#L726

The constraints are built by iterating deserialized object ConstraintConnectiveDescr descr, which is in a tree structure. However, this iteration only iterates the first layer of descr. All '&&' condition could be expanded into the first layer list but '||' could not. This caused that '||' expression could not parsed by MVELConstraint and the constraintType is UNKNOWN.

My code use constrains to find out which fields are being used.

Since '||' (double pipe) expression could not be built in MVELConstraint correctly, may I ask if there is any recommendation on how to find which fields are being used?

Thanks,

Leo

[Toshiya Kobayashi]

Hi Leo,

(value1 == 1 || value2 == 2) is not decomposed into finer constraints, so MVELConstraint (value1 == 1 || value2 == 2) is the expected result. Drools will let Mvel the expression execute.

> My code use constrains to find out which fields are being used.

I think parsing `MVELConstraint.expression` (-> value1 == 1 || value2 == 2) by yourself is the only way to detect the property names.

Regards,
Toshiya

2024年4月19日金曜日 6:42:39 UTC+9 zjli...@gmail.com:

[Leo Lin]

Hi Toshiya,

Thank you for your quick response.

May I ask if you have any recommendation on how to parse ` `MVELConstraint.expression` (-> value1 == 1 || value2 == 2)`?

String parsing seems burdensome as we need to consider different cases(e.g, 3 and more fields, or multiple parenthesis) and then map them back to the object.fields().

May I ask if Drools has any library to support the parsing? Or if we could transform it into other type of Constraints which has a finer Constraint?

Thanks,

Leo

[Toshiya Kobayashi]

A) If you just want to collect property names, simple String parsing may be enough. You would just read from start to end and pick java identifiers, ignoring String/char literals (surrounded by " or ') and skipping other chars (e.g. operators, parenthesis).

B) Drools internally uses antlr to parse DRL. You can make use of the parser class, which actually decomposes such a "||" constraint into finer elements at the first stage.

You can try like this:

```

DrlExprParser exprParser = new DrlExprParser(LanguageLevelOption.DRL6);
ConstraintConnectiveDescr constraintConnectiveDescr = exprParser.parse("value1 == 1 || value2 == 2");
List<BaseDescr> childDescrs = constraintConnectiveDescr.getDescrs();
...

```

Note that DrlExprParser is an internal class, so it may be changed in the future. If you want to write your own parser class, https://github.com/kiegroup/drools/blob/main/drools-drl/drools-drl-parser/src/main/resources/org/drools/drl/parser/DRL6Expressions.g is the antlr parser definition.

Regards,

Toshiya

2024年4月20日土曜日 1:15:08 UTC+9 zjli...@gmail.com:

[Leo Lin]

Thank you for answering the question! The answers are pretty informative and helpful!

I will try to leverage DrlExprParser on parsing.

Thanks,

Leo

[Luca Molteni]

> On 22 Apr 2024, at 06:29, Leo Lin <zjli...@gmail.com> wrote:
>
> Thank you for answering the question! The answers are pretty informative and helpful!
> I will try to leverage DrlExprParser on parsing.
>
> Thanks,
> Leo

If you want to get into Drools parsing, we kind of had to parse everything to build the executable model, so by taking a look at the internals of the executable model compiler you’ll find something you might need.

For example while compiling a MVEL expression with the MVEL to Java compiler you can find the used properties in this field

https://github.com/apache/incubator-kie-drools/blob/18047e93c81c3502cd759fbe3e8ce0c8f2e796f4/drools-model/drools-mvel-compiler/src/main/java/org/drools/mvelcompiler/PreprocessPhase.java#L62

Luca

[Leo Lin]

Thank you Luca!