[rules-users] Maps in Drools

[André Thieme]

KDR schrieb:
> Hi, I'm relatively new to both Java and Drools. I'm trying to figure out how
> to use maps in Drools. I've looked at the thread
> http://www.mail-archive.com/rules-users@lists.jboss.org/msg09802.html
> 
>>From what I've read generally it seems best to insert objects directly
> rather than use nested accessors. So I've been experimenting with trying to
> insert a map and then checking stuff in it.
> 
> I set up a simple test map of String to Integer, with just "a" as key and 1
> as value, and "b" with 2.
> Map<String, Integer> map = new HashMap<String, Integer>();
> map.put("a", 1);
> map.put("b", 2);
> String a = "a";
> 		    
> I then inserted the map and also inserted the String a of value "a".
> 
> Here's the test rule, with various things I tried commented out:
> 
> rule "testing maps"
> 	dialect "mvel"
> 	when
> 		$str: String()
> 		// $m: Map( this[$str] == 1 ) # error
> 		// $m: Map( this.$str == 1 )   # error
> 		// $m: Map( this["$str"] == 1 ) # compiles but rule won't fire
> 		$m: Map( this["a"] == 1 ) # this works however!
> 	then
> 		System.out.println($m[$str]); #also works with String and Map objects & no
> conditions
> end
> 
> It obviously doesn't like it when I try to use the String object as the key
> for the map. But it works when I use a String literal as the key. What am I
> doing wrong?
> 
> Does anyone have any suggestions please, or shall I give up and either use
> eval as mentioned in
> http://www.mail-archive.com/rules-users@lists.jboss.org/msg09716.html or use
> the map as a field of another object which I insert instead of the map (in
> fact that was my original plan!)?

In exactly this thread Marc answered to that problem.
http://www.mail-archive.com/rules-users@lists.jboss.org/msg09837.html

His first idea is that this is a bug in the mvel dialect.
I tried exactly that. I had a global var and put the string "a" into it.
Then I wanted to check if Map( this[myVar] == 1 ) but this didn't work.


> I'd also need to test for null i.e. whether a key/value pair exists for a
> given String as the key.

This seems to be only true when you go the route that I go, namely using
the default rule syntax, i.e., eval.


Sunny greetings,
André
-- 
Lisp is not dead. It’s just the URL that has changed:
http://clojure.org/