[rules-users] Why is a flat object recommended?

[Libor Nenadál]

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. :(
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