[rules-users] Clarification of using collections as facts

[Edson Tirelli]

   Hi all, sorry for the long absence. I'm back, but I know no one noticed,
anyway... ;)

   Allen, not really. Collections usually are not asserted as standalone
facts because they lack semantic meaning. They are abstractions, if you know
what I mean.

   So, in your case, you can write patterns like this:

Foo( names contains "abc" )
Foo( names not contains "abc" )

    Now, a regexp will act over Strings, not collections. So if you want to
write a rule that acts on each name that matches a regexp, you must
"iterate" over your collections elements using the "from" CE:

Foo( $names : names )
String( this matches "error.*" ) from $names

    Now, if you want the rule to re-create a new list containing only the
names matching a pattern, THEN you may use the collect CE to create a
collection:

$foo : Foo( $names : names )
$foundThese : ArrayList( ) from collect( String( this matches "error.*" )
from $names )

   You may also filter your resulting collection to only fire the rule when
there are actual elements. Just for the sake of examples, since you are
already binding $foo, you don't need to bind $names too:

$foo : Foo( )
$foundThese : ArrayList( size > 0 ) from collect( String( this matches
"error.*" ) from $foo.names )

    Happy drooling,
          Edson


2008/7/15 Bagwell, Allen F <afbagwe at sandia.gov>:

>
> I'm going through the Drools documentation regarding the use of lists and
> other collections. I want to make sure my understanding is correct.
>
> So if I have a class:
>
> public class Foo {
>
>    private int x;
>    private int y;
>    private List<String> names = ArrayList<String>();
>
>    public List<String> getNames() {return names;}
>
>    // appropriate getters/setters for the int fields ...
> }
>
> With this I can insert a Foo object into working memory. I can even make
> the int fields dynamic facts with the appropriate addition of "bean-ifying"
> code.
>
> For the List, however, I need to insert it separately into working memory
> in order to make use of Drools' rule language ('contains' ,etc.)? I'm
> assuming that doesn't come free because I have to insert my own custom class
> objects if they are included via composition in a larger fact!
>
> Also, I'm guessing the Collection type classes cannot be dynamic facts?
> Meaning if I modify my List with add, remove,  clear, etc., I must
> explicitly call update in my code on the List fact in order to alert Drools
> to a change in the List, correct?
>
> So then the question also comes up as to how I would write a rule that
> looks for all instances of Strings in the List that match against a given
> regex expression. I've not been successful at figuring this out...
>
> Rule "when a Foo List is modified, get all Strings in it that start with
> 'error'"
> when
>    $foo : Foo($names : names)
>    $foundThese : ArrayList() from collect(??? matches "error.*" from
> $names)
> then
>    // act on $foo and $foundThese
>
> I know that rule is NOT correct, but I'm not sure how I'm supposed to
> access an immutable object inside a collection. The examples in the
> documentation show how to get at mutable class objects.
>
>
> Thanks!
> -Allen
>
>
>
>
>
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>
>


-- 
Edson Tirelli
JBoss Drools Core Development
JBoss, a division of Red Hat @ www.jboss.com
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