old_contents: # Copyright 2021 Berezi - AvanzOSC
  # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
  from odoo import models, fields
  
  
  class EventRegistration(models.Model):
      _inherit = 'event.registration'
  
      user_id = fields.Many2one(
          string='User', comodel_name='res.users', related='student_id.user_id',
          store=True)
      signup_url = fields.Char(
          string='Sign Up URL', related='user_id.signup_url', store=True)
  
new_contents: # Copyright 2021 Berezi - AvanzOSC
  # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
  from odoo import models, fields, api
  
  
  class EventRegistration(models.Model):
      _inherit = 'event.registration'
  
      user_id = fields.Many2one(
          string='User', comodel_name='res.users', compute='_compute_user_id',
          store=True)
      signup_url = fields.Char(
          string='Sign Up URL', related='user_id.signup_url', store=True)
  
      @api.depends('student_id')
      def _compute_user_id(self):
          for registration in self:
              registration.user_id = registration.student_id.user_ids[:1]
  
subject: solve user. (#1280)