I have a computed field that computes every time I refresh the page. I tried using api.depends but it computes twice, once when I fill in the dependent field and once when I save. I want to compute the once then never change it again for that record. How can I do that? I have included the code below 

    

    product_name = fields.Many2one('product.info', string='Product')
    units_sold = fields.Integer(string='Units Sold', required = True)    
    current_stock = fields.Integer("Today's Stock", compute='_compute_stock', store=True)    
    units_entered = fields.Integer('Stock Entered')



    @api.depends('units_sold')    
    def _compute_stock(self):        
        for rec in self:             
            if rec.units_sold > 0 and rec.units_entered > 0:                
                rec.current_stock= rec.product_name.product_quantity - rec.units_sold + rec.units_entered            
            elif rec.units_sold > 0 and rec.units_entered == 0:                
                rec.current_stock= rec.product_name.product_quantity - rec.units_sold            
            elif rec.units_sold == 0 and rec.units_entered > 0:                
                rec.current_stock= rec.product_name.product_quantity + rec.units_entered            
            rec.product_name.product_quantity = rec.current_stock