I was hoping the recent conversation
regarding maps this would come up, but so far it hasn’t.
How do I check for conditions on the
element referenced in the map?
So given the following scenario:
SearchItem is the item being searched for
by the user
Item has the attributes: UPC (String) and
OnSale (Boolean)
StockRoom has an attribute of stock which
is Stock = HashMap<String, Item> (key value is UPC)
Now let’s say I want to write a rule
that says, “If the provided Item is in Stock, and on
rule “deal finder”
when
SearchItem($upc : upc != null) // see
if a UPC was provided
StockRoom($stock : stock) // get a
handle to the stock
HashMap (this[$upc] != null) from
$stock // look for the UPC in stock
Item (onSale == true) from $item //
Where do I obtain the reference handle that is $item?
then
// Buy stuff
End
What I’m trying to understand syntactically
is how do I obtain a reference handle to the “Item” found in the
HashMap reference (IE: $item) so that I can check additional attributes on
that Item.
I haven’t found any good examples of
how to do this yet.
From:
rules-users-bounces@lists.jboss.org
[mailto:rules-users-bounces@lists.jboss.org] On
Behalf Of Edson Tirelli
Sent: Friday, August 21, 2009 9:16
AM
To: Rules Users List
Subject: Re: [rules-users] Maps in
Drools
2009/8/20 André Thieme <address.good.until.2009.dec.14@justmail.de>
Would there not be an addition to the syntax needed, for the default
rule language? For mvel it would not require a change I guess.
No, as I mentioned to you, the idea is for the DRL to remain the same, so that
the rules author does not have to worry about what the IT guys are doing with
the domain model. So, the rules author would write:
Customer( name == "bob" )
The IT guy would simply use a configuration to tell the engine: this object
type uses a map format, that one is a POJO, that other is an XML entity, etc.
For instance, if he wants to do that in DRL (he could also use API, or conf
files), he could do:
declare Customer
@format( map )
end
declare Order
@format( pojo )
end
So, we have a clear distinction between the technical aspects and the business
aspects of the application.
Your clojure macro would generate always the same DRL code "Customer( name
== "bob" )", but you said yourself that clojure is 100% java
compatible, so imagine the enterprise had a domain model implemented as pojos
already and as part of the new application some new entities are modeled in
clojure. The macro would generate always the same code, but you would configure
some entities in the domain as POJOs and other entities as Maps.
Hmm, but the MVEL syntax can not magically eliminate the eval. Under the
hood the map accesses will still be inside an eval. Marc confirmed that
a few days ago.
MVEL only hides this from the user. This is what I will also do.
But under the hood it will become
$a:Map()
$b:Map()
eval( $a.get("type") == "Customer" )
eval( $b.get("type") == "DailyOrders" )
Here I think we have other misunderstanding. I will try to explain, but ideally
you need to learn a bit about the Rete algorithm to see the whole picture.
There are 2 types of eval(). Inline eval() and top level eval(). What you wrote
above is a top level eval, meaning it will become a node in the rete network.
So your example above generates an "execution plan" (making an
analogy with SQL) that will get all Maps in the working memory, join them in
tuples size 2, and then test each tuple for the 2 evals. So, you see why this
will generate C(n,2) partial matches, while C(n,2) as we know is n!/(n-2)!,
what is really bad for growing "n".
Now, the same thing could be written using inline evals as:
$a:Map( eval( $a.get("type") == "Customer" ) )
$b:Map( eval( $b.get("type") == "DailyOrders" ) )
In this case, the inline eval() will generate an alpha constraint in the
network, i.e., it will be applied BEFORE the joins. So, instead of doing all
combinaions possible between all maps as above, it will first find all Customer
maps and all DailyOrders maps and only after that will make a join between
them. So you get Customers * DailyOrders partial matches. The above evals are
semantically equivalent as:
$a:Map( this[ "type" ] == "Customer" )
$b:Map( this[ "type" ] == "DailyOrders" )
> But currently I am forced to produce this cross product, as there is no
direct support for Maps yet.
I hope that by the above you see that the problem of the cross products is not
a problem with Maps support, but rather a question of how to write better
rules. The same way you can write 2 completely different SQL queries that
return the same result but one is fast and the other is completely heavy and
slow, you can also write good rules and really bad rules.
[]s
Edson