You can do this, set it up relationally, this is the "best practice" way. This is what I told someone else:

Nested lists and hashmaps aren't good things for rules. Instead what you should do is break them apart and use a relational object to drive this. Take at look at the conways game of life example in MR2. There you'll see I have a grid of Cell objects, in the old version each Cell had a HashSet of neighbour Cells - this setup doesn't work too well with Rule Engines. Instead now I have a Neighbour class that acts as a relation, so I assert all my Cells and then I assert a Neighbour class for each neighbour pair I create. This creates a cross product, which you can explioit to drive your application. Basically think relationally, not nested structures, it's exactly the same problem if you wanted to represent this information in a database.

Mark

Anirvan Majumdar wrote:

Yeah,

I'd second that. Mark's approach actually had me all dazed right from the second line. I need to use Drools to accomplish a rather simple task, however the suggestions made by Mark had me all lost.

My requirement is that I have an input XML which my application should parse and output another XML. The output XML will have tags and values capturing the required business model. The structure of the input XML is something like this:

    <parent name="aaaaaaa">
           <child value="sdfsdfsdf"/>
           <child value="sdfsdfsdf"/>
           <child value="sdfsdfsdf"/>
           <child value="sdfsdfsdf"/>
    </parent>

Now the transformation of this input XML has to be done condition to certain rules which our client will provide us. I thought Drools would fit in very well. However after reading and acquiring a basic understanding I came across this particular limitation.

My problem arises since, the input bean's parent tag can have varying number of child tags. As a consequence, I can't define a single Bean class to capture this XML data. I can capture the data in a HashMap, but then I can't access the key-value pairs from the "when" block of a rule.

Arjun, as you suggested,
- can you tell me whether I can really utilize DynaBeans to meet my ends in this particular case?
- Can you also let me know a simple approach I can adopt?
- also, if eval can be used in this case, can you pass across an example to show how to access say, the key-values from a HashMap set in the asserted Bean object.

Amongst the other Rule Engines, I still think that Drools fits in best. A little help perhaps?

Thanks a lot!
Anirvan

On 5/15/07, Arjun Dhar <dhar_ar@yahoo.com> wrote:
Mark Proctor <mproctor <at> codehaus.org> writes:

>
>
> You need to alter the parser and the Extractor api, you also need to be
> able to deal with ShadowFacts, where we need to know the previous and
> current value.Further to that nested value should not change without
> notifying the parent fact in the network - i.e. correct network state
> must always been maintained. It's a complex area, we are looking at a
> work around that allows this declarative language, but rewrites it as
> an eval - so you don't get the performance advantages - but it's a
> quick work around for now.
> Mark

If what you want is the ability to extend the use of the class beyond what it
was designed for (like a Dyna Bean, I think it is that!), then as a work around
you could use 'eval' over your object methods also, but this practice is
discouraged for a few reasons.

The other more sophisticated approach is already given by Mark!

regards,
Arjun

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