[web2py] setting db table for SQLFORM from an argument

[monotasker]

I'm trying to abstract several content creation controllers into one 
controller. I want to use request.args[0] to set the db table in which the 
new row will be created. So far I've got this:

def add():
    the_table = request.args[0]
    tb = "db.%s" % the_table
    form = SQLFORM(tb)

but I'm getting an error that "str object does not have the attribute 
'writeable'". It seems like SQLFORM is treating 'tb' as a string object 
rather than taking the string's content as an argument. Am I missing 
something? Is there a better way to do this?

Thanks,

Ian