[rules-users] MVEL and Maps

[Wolfgang Laun]

For the sake of clarification: Are you saying that there is, or should be,
some magic which would make a constraint like
   Map ( this["departureDate"] == "07-Jan-2009" )
behave as a comparison between two java.util.Date values if the LHS Object
is of this type? And also if it is a GregorianCalendar object?

Since even you are "not sure" regarding MVEL: What is the definite reference
for MVEL in Drools? I have a vague recollection of reading somewhere that
not all of MVEL is available...

-W


2009/11/8 Edson Tirelli <ed.tirelli at gmail.com>

>
>      Ok, without discussing the merits of using (or not) maps, the reason
> you are having problems with dates is that the "string-based" date is syntax
> sugar in Drools parser/compiler. Although, when you create expressions using
> nested accessors or [] for collection/map element access, drools wraps the
> whole thing into an "eval" and delegates to MVEL. We do some transformations
> to bridge the gap between drools operators and mvel operators, but I don't
> think drools has anything to "transform" the string-based date into
> something mvel understands.
>
>      Having said that, we should probably add this functionality to the
> "MVELDumper" class for better integration. Would you please open a jira for
> that? Also, patches are welcome in case someone would like to help. This is
> an isolated part of the code and easy to understand.
>
>     Meanwhile, best you can do is avoid the use of this Map syntax +
> string-based dates, or write your expressions directly in a way the mvel
> understands (not sure mvel has a special syntax for dates).
>
>     Same explanation for the NPE.
>
>     Edson
>
> 2009/11/8 Leonardo Gomes <leonardo.f.gomes at gmail.com>
>
> Hi Wolfgang,
>>
>> Here's the context:
>>
>> We're replacing a very basic home-made rule engine with drools. Today,
>> fact data can be added dynamically b/c for all rules our fact is
>> nothing more than a Map and we can register new fact data through an
>> UI where you say what's the key and what's the type of the value.
>> Rules can also be created dynamically and operate against those newly
>> created attributes of my fact.
>>
>> Incoming requests are key / value structures and we have a service
>> that will look at the list of registered fact data and try to find
>> that information in the incoming request by looking for the keys and
>> getting the corresponding values, then populating the Map I just
>> talked about which will be injected in the working memory for
>> evaluation against our rules.
>>
>> That's why, in Drools, I need to have my rules accessing data from a
>> Map. By using MVEL, as I said, it works fine with strings, integers
>> and list of strings, but I'm having problems with dates and would like
>> some advice on that case.
>>
>> Here's another example of things that work:
>>
>> rule "test rule"
>>        when
>>                 Map ( this["airportCode"] == "GIG"
>>                        , this["numberOfPassengers"] > 1
>>                         , this["class"] in ("BUSINESS", "FIRST CLASS") )
>>        then
>>                m.addMessage("rule matched");
>> end
>>
>> And here's where I have problems,
>>
>> Map ( this["departureDate"] ) == "07-Jan-2009" )
>>
>>
>> Thanks a lot,
>> Leo.
>>
>>
>>
>> On Sun, Nov 8, 2009 at 10:56 AM, Wolfgang Laun <wolfgang.laun at gmail.com>
>> wrote:
>> > On Sat, Nov 7, 2009 at 6:00 PM, Leonardo Gomes <
>> leonardo.f.gomes at gmail.com>
>> > wrote:
>> >>
>> >> Hello,
>> >>
>> >> I'm feeding my working memory with Maps (unfortunately, this is a
>> >> requirement and I can't use beans).
>> >>
>> >
>> > Before people start racking their brains to overcome all the resulting
>> > difficulties: Could you please explain what this requirement is meant to
>> > achieve?
>> > -W
>> >
>> >
>> >>
>> >> To access the values in this maps I use the following MVEL expression :
>> >>
>> >>                Map ( this["airportCode"] == "GIG"
>> >>                        , this["numberOfPassengers"] > "1"
>> >>                        , this["class"] memberOf ("BUSINESS", "FIRST") )
>> >>
>> >> And it works fine, except for dates where I get a ClassCastException,
>> >> even tough I'm using the right pattern:
>> >>
>> >> Map ( this["departureDate"] > "07-Nov-2009" )   --- fails
>> >>
>> >> Is there anything I need to do to make the dynamic cast of dates work,
>> >> in this particular case?
>> >>
>> >> --
>> >>
>> >> Morever, I get NPEs if there's no value for a given key. For example,
>> >> the expression Map ( this ["name"] == 'Leonardo' ) would fail if
>> >> there's no entry with "name" as the key.
>> >>
>> >> Any help is welcome!
>> >>
>> >> Thanks in advance,
>> >> Leo.
>> >> _______________________________________________
>> >> rules-users mailing list
>> >> rules-users at lists.jboss.org
>> >> https://lists.jboss.org/mailman/listinfo/rules-users
>> >
>> >
>>
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>
>
>
> --
>  Edson Tirelli
>  JBoss Drools Core Development
>  JBoss by Red Hat @ www.jboss.com
>
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