What Not to Do (a cautionary tale)
- From: "Gavin Kistner" <gavin.kistner@xxxxxxxxx>
- Date: Thu, 7 Sep 2006 03:13:31 +0900
I wrote a little code generation system for my company. It's a small
script that reads in an XML file and an ERB template, feeds the XML file
to the template, and writes out the result to a file.
To help those in the company who don't know XPath, I extended REXML with
a couple convenience methods that let you search for elements by name
and/or attributes.
class REXML::Element
# Find all descendant nodes with a specified tag name and/or
attributes
def find( tag_name='*', attributes_to_match={} )
self.each_element(
".//#{REXML::Element.xpathfor(tag_name,attributes_to_match)}" ){}
end
# Find all child nodes with a specified tag name and/or attributes
def kids( tag_name='*', attributes_to_match={} )
self.each_element(
"./#{REXML::Element.xpathfor(tag_name,attributes_to_match)}" ){}
end
...
end
This worked very nicely. Template writers could simply do something
like:
In this file I see the following classes: <%=
root.find( 'class' ).map{ |el| el.attributes[ 'name' ] }.join( ', '
)
%>
All was fine under 1.8.4.
I just upgraded to 1.8.5 and all templates using #find broke. After a
bit of sleuthing, it looks like REXML changed the Document#root method
in a way that caused it to call Elements#[], and that method uses
@element.find, mixed in from Enumerable. My Element#find was overriding
Enumerable#find, and all hell broke loose as a result.
Moral of the story (pick one or more):
[ ] Don't extend someone else's class with common method names
[ ] Don't upgrade your Ruby environment unless you need to
.
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