In Openerp 7 Functional field added with store in Database

I used the below code:-

    def _concat_attached_file(self, cr, uid, ids, name, arg, context=None):
        res = {}
        attachment_obj = self.pool.get('ir.attachment')
        for id in ids:
            procedure_attachment = attachment_obj.search(cr, uid, [('res_model', '=', 'model_name'), ('res_id', '=', id)], context=context)
            filelist = []
            for val in attachment_obj.browse(cr, uid, procedure_attachment):
                filelist.append(val.name)
            filevalue = ', '.join(filelist) 
            if not filevalue:
                filevalue = None            
            res[id] = filelist
        return res

  'attached_file'   : fields.function(_concat_attached_file, method=True, store=True,  string='Attached File Name', type='char'),

But the above functional fields generate output only with Store=False. Suppose used Store=True output not generated. I want to store the db also how to fix ?Thanks