Java pojo's nested fields is just a weak way to represent relations
between objects. If you want to exploit those properly in a rule engine,
best to use real relations.
Mark
Libor Nenadál wrote:
Mark Proctor wrote:
> We can exploit cartesian products and indexing for == constraints. If
> its a nested model we have to iterate over all possible instances. The
> other problem is if the nested model changes the engine has no idea this
> has happened, which if you are not careful can lead to data integrity
> issues.
>
>
I think that plain flat model is too limited for real life. In fact in most
situations you just cannot avoid nesting.
For example when I try to search for cars with 4 cylinder engine the best
approach I found is:
rule "get 4 cylinder cars"
when
Car ( $e: engine != null ) // car without engine? where are we? :)
Engine ( cylinderCount == 4 ) from $e
then
System.out.println("Found 4 cylinder car.");
end
This should be quite effective. I doubt that the following solution with
cartesian product has the same complexity:
rule "get 4 cylinder cars"
when
$e: Engine ( cylinderCount == 4 )
Car ( engine == $e )
then
System.out.println("Found 4 cylinder car.");
end
Using approach Car ( engine.cylinderCount == 4 ) tends to raise
NullPointerException and I had problems with assigning the cylinderCount to
variable so this was not a way to go for me.
But this is not much a problem for me as I am a programmer and I can
understand the rule language and data structures. But when business users
are to edit rules (using Guvnor as the tool of choice) we come to troubles.
I think that guided editor for rules does not allow "from". More importantly
- can you create such a structure for a test scenario? I failed. :(