old_contents: # -*- coding: utf-8 -*-
  # Part of Odoo. See LICENSE file for full copyright and licensing details.
  
  from odoo import fields, models
  from odoo.addons.base.res.res_partner import WARNING_MESSAGE, WARNING_HELP
  
  
  class ResPartner(models.Model):
      _inherit = 'res.partner'
  
      sale_order_count = fields.Integer(compute='_compute_sale_order_count', string='# of Sales Order')
      sale_order_ids = fields.One2many('sale.order', 'partner_id', 'Sales Order')
      sale_warn = fields.Selection(WARNING_MESSAGE, 'Sales Order', default='no-message', help=WARNING_HELP, required=True)
      sale_warn_msg = fields.Text('Message for Sales Order')
  
      def _compute_sale_order_count(self):
          SaleOrder = self.env['sale.order']
          for partner in self:
              partner.sale_order_count = SaleOrder.search_count([
                  ('partner_id', 'child_of', partner.ids)
              ])
  
new_contents: # -*- coding: utf-8 -*-
  # Part of Odoo. See LICENSE file for full copyright and licensing details.
  
  from odoo import fields, models
  from odoo.addons.base.res.res_partner import WARNING_MESSAGE, WARNING_HELP
  
  
  class ResPartner(models.Model):
      _inherit = 'res.partner'
  
      sale_order_count = fields.Integer(compute='_compute_sale_order_count', string='# of Sales Order')
      sale_order_ids = fields.One2many('sale.order', 'partner_id', 'Sales Order')
      sale_warn = fields.Selection(WARNING_MESSAGE, 'Sales Order', default='no-message', help=WARNING_HELP, required=True)
      sale_warn_msg = fields.Text('Message for Sales Order')
  
      def _compute_sale_order_count(self):
          sale_data = self.env['sale.order'].read_group(domain=[('partner_id', 'child_of', self.ids)],
                                                        fields=['partner_id'], groupby=['partner_id'])
          # read to keep the child/parent relation while aggregating the read_group result in the loop
          partner_child_ids = self.read(['child_ids'])
          mapped_data = dict([(m['partner_id'][0], m['partner_id_count']) for m in sale_data])
          for partner in self:
              # let's obtain the partner id and all its child ids from the read up there
              partner_ids = filter(lambda r: r['id'] == partner.id, partner_child_ids)[0]
              partner_ids = [partner_ids.get('id')] + partner_ids.get('child_ids')
              # then we can sum for all the partner's child
              partner.sale_order_count = sum(mapped_data.get(child, 0) for child in partner_ids)
  
subject: use read_group instead of search_count