class delais(models.Model):

 _name = 'delais.delais'
_description = 'delais.delais'

project_ids = fields.Many2one('project.project', string='Projet')

 delai_arr_mois = fields.Float(string='Délais D arrét en mois', compute='_delai_arret')

this function calculates the sum of the number of missions per project

@api.multi
def _delai_arret(self):
    domaine = ('project_ids', '=', self.project_ids.id)
    print(domaine)
    dict = self.env['mission.mission'].search_read([domaine], ['nbr_mission'])
    print(dict)
    if not dict:
        pass
    else:
        somme = 0
        for key in dict:
            print(key['nbr_mission'])
            somme = somme + key['nbr_mission'] / 30
            print('la somme est : {somme}')
            self.delai_arr_mois = somme
            print(somme)

• class Missions(models.Model):
  
      _name = 'mission.mission'
      _description = 'mission.mission'
  
      _rec_name = 'mission'
  
      project_ids = fields.Many2one('project.project', string='Projet')
     ('Déf', 'Définitif')])
      ordre_arre = fields.Date(string='Ordre d arrét')
      ordre_repr = fields.Date(string='Ordre de reprise') 
   
      nbr_mission = fields.Float(string='nombre de mission', compute='_delais_mission')
  
  this function calculate the number of missions
  
  
  def _delais_mission(self):
  for k in self:
  
  if k.ordre_arre and k.ordre_repr:
   date_1 = datetime.strptime(k.ordre_arre, '%Y-%m-%d')
   date_2 = datetime.strptime(k.ordre_repr, '%Y-%m-%d')
   k.nbr_mission = ((date_2 - date_1).days) / 30
  #print(k.nbr_mission)
  
  error is : 
  
   raise ValueError("Expected singleton: %s" % self)
  ValueError: Expected singleton: delais.delais(1, 4)