[VPM] converting objects to/from xml

[Peter Scott]

At 05:40 PM 7/12/2003 -0700, Darren Duncan wrote:
>I have another question, mainly for those of you who have worked with 
>XML in Perl.
>
>Given that my SQL::ObjectModel objects are conceptually organized like 
>XML, I would like to make a module which will convert them to XML, and 
>conversely parse an XML document into one of those objects.
>
>I would like to use some suitable modules from CPAN which handle most 
>of the work, but I am unsure of which ones to use.  I had tried 
>looking at some SAX modules (which seem common), because a callback 
>interface seems to make sense for parsing XML, but I got the 
>impression that this was perhaps too complicated.  Most of the SAX 
>examples I saw didn't have much to do with parsing or generating 
>XML.  I want object-oriented modules, and pure perl also, for the moment.
>
>I am wondering whether I would be better off coding this conversion 
>myself, which I would expect is less than 40-100 lines of code, or 
>whether the robustness or wider compatability brought by a CPAN module 
>would be better.
>
>I prefer a simple solution, so I could for example do this:
>
>my $firstxml = "...";
>
>my $objectmodel = SQL::ObjectModel::XML->from_xml( $firstxml );
>
>my $secxml = SQL::ObjectModel::XML->to_xml( $objectmodel );
>
>Then "$firstxml eq $secxml" probably returns true.
>
>The hypothetical SQL::ObjectModel::XML module would have all the 
>references to the CPAN module or alternately home grown implementation 
>internally.
>
>Thanks in advance for any suggestions. -- Darren Duncan

O'Reilly's "Perl & XML" is excellent for this kind of thing.  From 
reading Chapter 6, I think you can do:

use XML::TreeBuilder;
my $tree = XML::TreeBuilder->new;
$tree->parse($firstxml);
my $objectmodel = SQL::ObjectModel->model_from_tree($tree);

Since I don't know what your object model is, I suggest you write the 
method model_from_tree() to convert the XML::Parser tree to whatever 
that is.  That tree is a mixture of arrayrefs and hashrefs, too long to 
type an example here.  Logically it should map cleanly to your object 
model.  Obviously you can encapsulate all the calls above into a single 
method of your own if you want.

Then for output you have:

my $secxml = SQL::ObjectModel->to_tree($objectmodel)->as_XML;

That last one is a bit harder because creating the tree requires making 
this hierarchy of XML::Element objects.  There are methods in that 
class that let you build up such a tree.

Disclaimer: I haven't done this.  But if I was going to do what you're 
describing, this is the approach I would try to make work.
-- 
Peter Scott
Pacific Systems Design Technologies
http://www.perldebugged.com/