title: ValueError Expected singleton many2many field
body: <pre><code>professor class
  </code></pre>
  <p>{# -<em>- coding: utf-8 -</em>-
  from odoo import fields, models, api
  from datetime import datetime</p>
  <pre><code>class GiProfesseur(models.Model):
      _name = &quot;gi.professeur&quot;
      _inherit = ['mail.thread']
      _description = &quot;Professeur du département de Genie Industriel&quot;
      _rec_name = 'combination'
  
      nom = fields.Char(string='Nom', required=True)
      prenom = fields.Char(string='Prenom', required=True)
      numero_de_carte_nationale = fields.Char(string='Numéro de carte nationale',     required=True)
      sexe = fields.Selection([('Masculin','Masculin'),('Féminin','Féminin')], required=True)
      date_de_naissance = fields.Date('Date de naissance' , required=True)
      Wilaya_de_residence = fields.Char(string='Wilaya de naissance', required=True)
      adresse = fields.Text(string='Adresse')
      specialite = fields.Many2many('gi.specialite', string=&quot;Spécialité&quot;, required=True)
      classroom = fields.Many2many('gi.classroom', string=&quot;Groupe&quot;)
      grade = fields.Char(string='Grade',)
      position = fields.Text(string='Position')
      date_recruitment = fields.Date(&quot;Date de recruitment&quot;, required=True, readonly=True, default=datetime.now().date())
      adresse_Email = fields.Text(string='Adresse E-mail', required=True)
      telephone = fields.Char(string='Téléphone', required=True)
      image = fields.Binary(string='Image')
      combination = fields.Char(string='Combination', compute='_compute_fields_combination')
  
      @api.depends('nom', 'prenom')
      def _compute_fields_combination(self):
          for Professeur in self:
              Professeur.combination = Professeur.nom + ' ' + Professeur.prenom}
  
  
  classroom class
  </code></pre>
  <p>{# -<em>- coding: utf-8 -</em>-
  from odoo import fields, models, api , _</p>
  <pre><code>class GiClass(models.Model):
      _name = &quot;gi.classroom&quot;
      _inherit = ['mail.thread']
      _description = &quot;les classroom du département de Genie Industriel&quot;
      _rec_name = 'nom'
  
      nom = fields.Char(string='Code du grupe', required=True)
      etudiants = fields.One2many('gi.etudiant', 'classroom', string='Etudiants')
      professeurs = fields.One2many('gi.professeur', 'classroom', string='Professeurs')
      cours = fields.One2many('gi.cours', 'classroom', string='Etudiants')
      examens = fields.One2many('gi.cours', 'classroom', string='Examens')
      num = fields.Char(string='Numéro du classroom', required=True)}
  </code></pre>
  <p>When I add two or three classroom to professor class</p>
  <pre><code>classroom = fields.Many2many('gi.classroom', string=&quot;Groupe&quot;)
  </code></pre>
  <p>I get this error</p>
  <pre><code>ValueError: Expected singleton: gi.classroom(12, 13)
  </code></pre>