old_contents: # -*- coding: utf-8 -*-
  # Part of Odoo. See LICENSE file for full copyright and licensing details.
  
  from odoo import api, fields, models
  
  class ResPartner(models.Model):
      _inherit = 'res.partner'
  
      event_count = fields.Integer("Events", compute='_compute_event_count', help="Number of events the partner has participated.")
  
      def _compute_event_count(self):
          for partner in self:
              partner.event_count = self.env['event.event'].search_count([('registration_ids.partner_id', 'child_of', partner.ids)])
  
      @api.multi
      def action_event_view(self):
          action = self.env.ref('event.action_event_view').read()[0]
          action['context'] = {}
          action['domain'] = [('registration_ids.partner_id', 'child_of', self.ids)]
          return action
  
new_contents: # -*- coding: utf-8 -*-
  # Part of Odoo. See LICENSE file for full copyright and licensing details.
  
  from odoo import api, fields, models
  
  class ResPartner(models.Model):
      _inherit = 'res.partner'
  
      event_count = fields.Integer("Events", compute='_compute_event_count', help="Number of events the partner has participated.")
  
      def _compute_event_count(self):
          for partner in self:
              partner.event_count = self.env['event.event'].sudo().search_count([('registration_ids.partner_id', 'child_of', partner.ids)])
  
      @api.multi
      def action_event_view(self):
          action = self.env.ref('event.action_event_view').read()[0]
          action['context'] = {}
          action['domain'] = [('registration_ids.partner_id', 'child_of', self.ids)]
          return action
  
subject: use sudo to compute event_count