title: Odoo12 TypeError
body: <pre><code>from odoo import models,fields
  
  # car/models/car.py
  class Car(models.Model):
     _name = 'car.car'
  
     name  = fields.Char('Car name', size=25)
     brand = fields.Char('Car brand',default='Citroën DS',size=25)
     country = fields.Char('Country name ',default='France',size=30)
  
     def __init__(self):
  
        return 'The car name is : %s,\nIts brand is : %s,\nIts manufactured country is : %s.\n' %    (self.name,self.brand,self.country)
  
  # car/models/product_template.py
  
  class ProductTemplate(models.Model):
     _inherits = 'product.template'
     _name     = 'product.template'
  
     car       = fields.Many2one('car.car', string='Car name', ondelete='SET NULL', auto_join=True)
  </code></pre>
  
  <p>My error is : Before to fill the car informations, in Sales/Products/Products, into tab 'General Information' i selected 'Storable Product' value in 'Product Type' field. Right now i am filling the car informations, 'save' successfully. I wanna edit the value of 'Product Type', from 'Storable Product' to 'Consumable' or 'Service', i got 
   this error :</p>
  
  <p>File "/odoo/odoo12/odoo/models.py", line 5384, in onchange
        for cmd in values[name]:
    TypeError: 'int' object is not iterable"</p>
  
  <p>Please helps.</p>