From: Robert Klemme <shortcutter@...>
Date: 2007-01-03T19:50:10+09:00
Subject: Re: Ruby classes and objects from XML

On 03.01.2007 10:57, Imobach Gonz�lez Sosa wrote:
> I have an XML file generated from TaskJuggler[1] and I'd like to have
> the objects described in that file (Projects, Resources, Tasks and
> Bookings) available as Ruby classes and objects, so I can write
> reports about the project.
> 
> In summary, I have and XML file (argh!) and I want to convert it
> "automagically" into Ruby classes ("struct"-like classes will suffice)
> and objects, without having to write a "parser".
> 
> Does exist any library/module that could do that job?
> 
> [1] http://www.taskjuggler.org/

These are your options as far as I can see at the moment:

1. use REXML DOM

You can read the file in a single line and get a document object.  You 
can then extract attributes via XPath or direct traversal.  In this case 
you need zero code for the parsing / reading but the extraction might be 
a bit more complex than you want (using XPath expressions is more 
complex than just using obj.some_attribute).

2. use REXML stream parser with Hash

You will have to code a generic listener to XML stream parsing events 
(not too complicated) that basically transfers data seen into a tree if 
Hash instances.  XML element names become Hash keys and nested elements 
become Hashes as values.  You probably need additional (fixed) keys for 
storing XML attributes and a link to the parent element.

Stream parsing:
http://www.germane-software.com/software/rexml/docs/tutorial.html#id2248594

3. use REXML stream parser with OpenStruct

Basically the same as option 2 but you use OpenStruct instead of Hash.

4. use REXML stream parser with report on the fly

Depending on your reporting needs you might not have to construct an 
object tree at all but can create your report while you go through the 
file.  This is the most efficient approach for large files.

5. use REXML stream parser with custom classes on the fly

You implement a listener for XML stream parsing events that will create 
custom classes on the fly, i.e. when it sees a "Person" it will create 
class Person; then when it sees a nested element it will create a 
attribute for it etc.  This is the most complex and think it's probably 
not worth the effort.

6. XML Mapping

Not sure whether that fits your needs or is stable at all:
http://raa.ruby-lang.org/project/xml-mapping/

Or this one
http://raa.ruby-lang.org/project/xmltreebuilder/

Other XML related libs:
http://raa.ruby-lang.org/cat.rhtml?category_major=Library;category_minor=XML

7. use an XSLT tool

You could as well use an XSLT tool to create your report as 
transformation of your input file.  Whether that works depends on your 
reporting requirements.


Depending on what kinds of reports you want to do, option 1 or 2/3 might 
be the most efficient.  Especially if you just want to do something like 
"task x has n sub tasks" that can be easily accomplished with XPath.

If those XML files are large, then you should go with one of the stream 
parsing approaches because they are more memory efficient.  Also you can 
filter the data while you go (i.e. ignore attributes and nested elements 
you are not interested in) and thus further optimize your app.  The most 
efficient is certainly 4 if it's feasible.

Note, there are other XML parsers for Ruby around.  I prefer to start 
wiht REXML and only convert to something else if performance is an issue 
simply because it's part of the standard distribution and has a nice 
interface.

Kind regards

	robert