Hi,

if your class related with a one product.product use many2one.

class my_class(osv.osv) :
{
    _name = "my_class"
    _description = "My Class"
    _columns = {
        'number': fields.integer('Qte'),        
        'product_id': fields.many2one('product.product', 'Product'),
    }
}

in methods create and copy ,

def create(self, cr, uid, values, context=None):    
            #get product            
            product_pool = self.pool.get('product.product')                 
            product_id = values['product_id']
            #write a new value to product
            product_pool.write(cr, uid, [product_id],
                               {'qty_available': values['number'],}
                               )      
            return super(my_class, self).create(cr, uid, values, context)

Thanks.