[rules-users] MVEL and Maps

[Edson Tirelli]

     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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