title: Save value from readonly field in odoo
body: <p>I have 3 fields on form field sum is readonly. After click on Save data from readonly field is not store in database.</p>
  
  <p>Example: </p>
  
  <pre><code>class myClass(models.Model):
  
  number_1 = fields.Integer(store=True,default=0)
  number_2 = fields.Integer(store=True,default=0)
  sum = fields.Integer(store=True)
  
  @api.onchange('number_1','number_2')
  def compute_sum(self):
      total = self.number_1 + self.number_2
      self.sum = total
  </code></pre>