Hello, 

I'm on odoo 15 and I'm trying to create a custom module, but I have an error that I can't fix:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_http.py", line 237, in _dispatch
    result = request.dispatch()
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 687, in dispatch
    result = self._call_function(**self.params)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 359, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/odoo/service/model.py", line 94, in wrapper
    return f(dbname, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 348, in checked_call
    result = self.endpoint(*a, **kw)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 910, in __call__
    return self.method(*args, **kw)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 535, in response_wrap
    response = f(*args, **kw)
  File "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 1326, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 1318, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/usr/lib/python3/dist-packages/odoo/api.py", line 464, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "/usr/lib/python3/dist-packages/odoo/api.py", line 451, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/odoo/models.py", line 6507, in onchange
    result['value'] = snapshot1.diff(snapshot0, force=first_call)
  File "/usr/lib/python3/dist-packages/odoo/models.py", line 6331, in diff
    result[name] = field.convert_to_onchange(self[name], record, {})
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 2845, in convert_to_onchange
    return super().convert_to_onchange(value._origin, record, names)
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 868, in convert_to_onchange
    return self.convert_to_read(value, record)
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 2821, in convert_to_read
    return value.id
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 643, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 301, in _handle_exception
    raise exception.with_traceback(None) from new_cause
AttributeError: '_unknown' object has no attribute 'id'

Here is the model : 

from email.policy import default
from odoo import models, fields, api, tools
from odoo.tools import date_utils, email_re, email_split, is_html_empty
from odoo.addons.phone_validation.tools import phone_validation
from datetime import date, datetime, timedelta

class WeglioProspect(models.Model):
    # ---------------------------------------- Private Attributes ---------------------------------

    _name = "weglio.prospect"
    _description = "School prospection by Weglio"

    # --------------------------------------- Fields Declaration ----------------------------------

    prospect_note = fields.Html("Annotation prospect")
    admission_step = fields.Selection([
        ('nouveau', 'Nouveau'),
        ('relancer', 'Relancer'),
        ('refusé', 'Refusé'),
        ('reporté', 'Reporté'),
        ('session d’admission', 'Session d’admission'),
        ('absent en session d’admission', 'Absent en session d’admission'),
        ('coaching', 'Coaching'),
        ('admis', 'Admis'),
        ('eleve', 'Eleve')], "Etape d'admission", required=True, default='nouveau')

    # ---------------------------------------------- Contact information need to copy in contact module ------------------------------------------------
    civility = fields.Selection([
        ('monsieur', 'Monsieur'),
        ('madame', 'Madame'),
        ('autre', 'Autres')], "Civilité", track_visibility='onchange')
    name = fields.Char(required=True)
    email = fields.Char("Mail", required=True)
    phone = fields.Char("Téléphone", required=True)
    # ------------------- Adresse ---------------------
    street = fields.Char()  # "Adresse"
    city = fields.Char()  # "Ville"
    zip = fields.Char()  # "Code Postal"
    country_id = fields.Many2one('res.country')  # "Pays"
    # ------------------- Info naissance ---------------------
    date_birth = fields.Date("Date de naissance")
    city_birth = fields.Char("Ville de naissance")
    country_birth = fields.Many2one('res.country', 'Pays de naissance')

    # ---------------------------- Info ----------------------------
    source_id = fields.Many2one('utm.source', "Source")
    nationality = fields.Many2one('res.country', 'Nationalité')
    vital_card = fields.Char("Carte Vital", track_visibility='onchange')
    social_system = fields.Selection([('urssaf', 'URSSAF'),
                                      ('msa', 'MSA')], 'Régime social')
    sport = fields.Selection([('oui', 'Oui'),
                                ('non', 'Non')], 'Sportif')
    handicap = fields.Selection([('oui', 'Oui'),
                                  ('non', 'Non')], 'Handicape')
    situation_before = fields.Selection([('scolaire', 'Scolaire'),
                                        ('prepa', 'Prépa'),
                                        ('apprentissage', 'Apprentissage'),
                                        ('etudiant', 'Etudiant'),
                                        ('contrat apprentissage', 'Contrat d\'apprentissage'),
                                        ('contrat de professionnalisation', 'Contrat de professionnalisation'),
                                        ('contrat aide', 'Contrat aidé'),
                                        ('cfa avant signature contrat apprentissage',
                                         'CFA avant signature d\'un contrat d\'apprentissage'),
                                        (
                                        'cfa sans contrat suite à une rupture', 'CFA sans contrat suite à une rupture'),
                                        ('stagiaire formation professionnelle', 'Stagiaire formation professionnelle'),
                                        ('salarie', 'Salarié'),
                                        ('recherche un emploi', 'recherche d\'un emploi'),
                                        ('inactif', 'Inactif')], 'Situation avant')  # voir avec Guillaume
    last_degree = fields.Char('Dernier diplôme préparé')
    last_class = fields.Char('Derniere classe')  # voir avec Guillaume
    # ------------------- Info suivi ---------------------
    date_last_raise = fields.Date('Dernière relance')
    count_last_raise = fields.Integer('Nombre de relance', compute="_compute_raise_date")

    date_previous_raise = fields.Date('Dernière relance')
    count_previous_raise = fields.Integer('Nombre de relance')

    active = fields.Boolean(string='Actif', default=True)
    # ------------------- Document ---------------------
    curriculum = fields.Binary("CV", help="Déposer votre cv")
    cv_name = fields.Char(compute="_compute_cv_name")  # Nom du fichier upload

    motivation_letter = fields.Binary("Lettre de motivation", help="Déposer votre lettre de motivation")
    motivation_name = fields.Char(compute="_compute_motivation_letter_name")  # Nom du fichier upload

    upload_test = fields.Binary("Test d'admission", help="Déposer votre test d'admission")
    upload_admission_file = fields.Binary("Dossier d'admission", help="Déposer votre Dossier d'admission")
    # ------------------- Après être admis ---------------------
    company_status = fields.Selection([
        ('avec entreprise', 'Avec entreprise'),
        ('a placer en entreprise', 'A placer en entreprise'),
        ('doit trouver une entreprise', 'Doit trouver une entreprise')], "Status d'admission entreprise")
    contract_status = fields.Selection([
        ('contrat en cours', 'Contrat en cours'),
        ('interruption contrat', 'Interruption contrat'),
        ('stagiaire de la formation', 'Stagiaire de la formation')], "Status du contrat")
    # Faire la vue des champs ci-dessous

    admission_file_complet = fields.Boolean()
    wait_doc = fields.Boolean()

    # ------------------------- Fields Other Module Declaration ---------------------------

    # Permet de définir si la liste affiche uniquement les leads ou les entreprises
    company_id = fields.Many2one('res.company', string='Entreprise', domain=[('is_company', '=', True)])
    formation_id = fields.Many2one('weglio.formation', string='Formation souhaité')
    partner_id = fields.Many2one('res.partner', string='Nom, Prenom', domain=[('is_company', '=', False)])
    # domain permet de preselectionner entreprise lors de la création de celle ci
    session_re_ids = fields.Many2many('calendar.event', 'weglio_prospect_event_1_rel', 'id', 'event_id', string="Recrutement", domain="[('session_type', '=', 'recrutement')]")
    session_co_ids = fields.Many2many('calendar.event', 'weglio_prospect_event_2_rel', 'id', 'event_id', string="Coaching", domain="[('session_type', '=', 'coaching')]")

    recrutement_names = fields.Char(string="Recrutement", compute="_compute_session_re_ids_names")
    coaching_names = fields.Char(string="Coaching", compute="_compute_session_co_ids_names")

    # ------------------------------------- Statistique  ----------------------------------------------
    # ------------------------------------- Onchange simple  ----------------------------------------------
    def _onchange_company(self):
        if self.admission_step != "admis":
            self.company_status = ""
            self.company_id = ""

    @api.onchange("admission_file_complet", "admission_step")
    def _onchange_admssion(self):
        if self.admission_file_complet:
            self.admission_step = "admis"

    # ------------------------------------- Compute simple  ----------------------------------------------

    @api.depends('session_re_ids')
    def _compute_session_re_ids_names(self):
        for prospect in self:
            prospect.recrutement_names = ', '.join(prospect.session_re_ids.mapped('name'))

    @api.depends('session_co_ids')
    def _compute_session_co_ids_names(self):
        for prospect in self:
            prospect.coaching_names = ', '.join(prospect.session_co_ids.mapped('name'))

    @api.depends('cv_name', 'partner_id', 'curriculum')
    def _compute_cv_name(self):
        if self.curriculum:
            for each in self:
                each.cv_name = "CV_" + each.partner_id.name
        else:
            self.cv_name = ""

    @api.depends('motivation_name', 'partner_id', 'motivation_letter')
    def _compute_motivation_letter_name(self):
        if self.motivation_letter:
            for each in self:
                each.motivation_name = "motivation_letter_" + each.partner_id.name
        else:
            self.motivation_name = ""

    @api.depends('partner_id')
    def _compute_contact_to_name(self):
        if self.name != "":
            for each in self:
                each.name = "Opportunité : " + each.partner_id.name
        else:
            each.name = "Opportunité : Anonyme"

    @api.onchange('date_last_raise')
    def onchange_date_derniere_relance(self):
        if self.date_last_raise: 
           self.admission_step = 'relancer'

#   Permet l'archivage des leads
    def toggle_archive(self):
        for record in self:
            record.is_archived = not record.is_archived

#   Permet +1 le nombre de relance
    @api.depends('date_last_raise', 'count_last_raise')
    def _compute_relance_date(self):
        for record in self:
            if record.date_last_raise != record.date_previous_relance: 
                record.count_last_raise = record.count_previous_raise + 1
            else:
                record.count_last_raise = record.count_previous_raise

            record.date_previous_raise = record.date_last_raise
            record.count_previous_raise = record.count_last_raise   

If anyone has an idea, I'm a taker.
Best regards