Documentation for lazy virtual fields incorrect in some cases

[Chris]

The docs for lazy virtual fields (http://web2py.com/book/default/
chapter/06#Virtual-Fields) give incorrect results; the virtual field
is incorrectly mapped to the last row selected in certain cases.

Here's a sample:

class table_story_titleAsSlug(object):
def titleAsSlug(self):
def lazy(self=self):
return urls.convert_to_slug(self.story.title)
return lazy
db.story.virtualfields.append(table_story_titleAsSlug())

titleAsSlug() will always return the value for the last row selected
from stories. While this may not apply in all cases, I've found a fix:

class table_story_titleAsSlug(object):
def titleAsSlug(self):
def lazy(title=self.story.title):
return urls.convert_to_slug(title)
return lazy
db.story.virtualfields.append(table_story_titleAsSlug())

Notes: In the debugger, the second self always refers to one instance
of table_story_titleAsSlug, which explains why self.story.title gets
overwritten. Are virtual field objects like table_story_titleAsSlug
only instantiated once?