[TM4J-users] Re: Problems with paramaterized prepared queries

[Kal A. <ka...@te...>]

Hi Bret,

On Thu, 2004-08-19 at 19:52, Bret Cohen wrote:
> Kal,
> 
> I would have been tempted to think what I've just
> found is a bug, but my past experiences have shown
> that the only bugs to be encountered are
> - the flu that I can't seem to permanently fight off
> - me  - bugging you about something that either turns
> out not to be a problem or that has an easy solution
> in retrospect. ;>
> 

All questions and bug reports are welcome - even if it turns out not to
be a bug, its a good indication of where the documentation needs beefing
up, and I think this is a good case in point (this is a long way of
saying "Keep asking!")


> Note: Example 8.1 of Using Replacement Values, in the TM4J Developer's Guide PDF (p. 64)
>       runs off of the side of the page, so it can't be read. When I cut & paste, I get more
> 	of it, but it still seems like parts are missing:
> 		QueryEvaluator qe = QueryEvaluatorFactory.newQueryEvaluator(tm);
> 		PreparedQuery pq = qe.prepareQuery("select $DESC from descendant-
> 		// Then later in the code, assume that we want to find the descendant-
> 		TologResultsSet rs = pq.execute(new Object[] { t });
> 
> (Do I detect the descendant example I first used to get the ball rolling? :>)
> 
Thanks for the documentation bug report - I guess I need to reformat the
code to get it to fit on the page.

> My confusion is over what sort of Objects may be passed to
> a prepared query to represent a Topic.
> 

This is not too clear from the documentation and should be - so thats
definitely a docs bug.

> For example, in my topic map, an activity Topic may be associated with
> sub-activity Topics (which are included in the activity).
> I want to use the parameterized prepared query to allow the user to enter
> the activity Topic for which sub-activity Topics are to be returned.
> 
> I'm trying to use the following prepared query:
> 	select $SUBACTIVITY_FOR from instance-of($ACTIVITY, activity),
> 	activity_component(%1 : includes, $SUBACTIVITY_FOR : included_in)?
> 
> The problem is that I think the query requires a Topic ID, a String, but
> when I attempt to pass the String giving a Topic ID into the query
> I get the following runtime exception:
> 
> java.lang.RuntimeException: error evaluating
> query.org.tm4j.tologx.TologProcessingException:
> Type mismatch in replacement of %1.
> A Topic was expected.
> 

Yes, you are expected to pass a Topic object in, so you can first find
the Topic that the user requested (using the normal TM4J indexes for
example) and then put that Topic object in the array of replacement
values that you pass to the method.

Under the covers, it is the *parser* that resolves all of those topic
reference types listed in the tolog language spec, and the predicate
implementations receive the Topic. When you use a replacement value, you
bypass the parser's string resolution part and the object you pass in is
passed straight to the predicate implementation - hence you need to pass
in the appropriate type of object (a Topic in this case).

I suppose that for completeness it might be nice if the replacement code
could do a String-to-WhatIsNeeded conversion for you and I'll look into
doing that as a feature enhancement, but the short-term solution is to
pass in the Topic, not the topic id string.

Hope this helps!

Cheers,

Kal

-- 
Kal Ahmed <ka...@te...>
techquila