[web2py] problem importing dbconnection - works on first request after server start then shows zero rows

[simon]

In my model file I am importing a database definition from a module file.
This works correctly the first time after the server is started showing the 
tablename and 11 rows in the table.
However for the second and subsequent requests it shows a rows object but 0 
rows in the table.
It works fine if I put the model definition code inline rather than 
importing.

MODEL:

import ocart2
web=ocart2.web
rows=web(web.category.category_id>0).select()
log.info((web.tables, rows))

MODULE:
from gluon import *
from config import webstring
web=DAL(webstring, pool_size=10, migrate=False, migrate_enabled=False)
web.define_table('category',
    Field('category_id', 'id', notnull=True, writable=False),
    Field('parent_id', 'integer', default=0, notnull=True, writable=False),
    Field('status', 'integer', default=1),
    Field('top', 'integer', notnull=True, writable=False))

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