[web2py] Category and Subcategory

[SeamusSeamus]

Right now I am categorizing my products by 'category' and 'subcategory' 
using smartgrid. This makes it very easy for me to log in and set my cats 
and sub cats. I have two tables below. 

db.define_table('cat',
    Field('name'),
    format = '%(name)s'
    )

db.define_table('subcat',
    Field('cat', 'reference cat'),
    Field('name'),
    format = '%(name)s'
    )

 I also have a third table for the product:

db.define_table('equipment',
...
    Field('category', db.cat),
    Field('subcategory', db.subcat, label="Sub-Category")
...
)

I am using this as my controller:

def productadmin():

    fields = [db.equipment.title, db.equipment.approved]

    grid = SQLFORM.grid(db.equipment, fields=fields)

    return dict(grid=grid)


Here is my question.
 When I click on 'edit' in the sqlform.grid for an item, and I want to 
change the category or assign the category, how do I make it so only the 
subcat will show if it is truly a subcat for the category? Currently, I 
have a list of all my cats and a list of all my subcats.


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