[web2py] Is there something like an "optional requires"

[Calycé]

Hi all,

I have created a table with a self reference as described below:
db.define_table('category',
    Field('parent_category', 'reference category'),
    Field('name', 'string', length=40, required=True),
    Field('description', 'text', required=True),
    Field('small_image', 'upload'))

Now I'd like to have an "optional requires" clause. For example, if I
add the following clause "store.category.parent_category.requires = 0
or IS_IN_DB(db, db.category.id, '%(name)s')" I have a nice combo
whenever I use the appdadmin to insert rows BUT I have to select
something from the combo. In my case, the parent_category field should
be optional. In extenso, categories without parent_category would be
root categories so selecting a parent_category is optional.

Is there a way to present a combo as does the "requires" but without
enforcing the field ? Do I have to create a specific form for that ?