You can use force_save in xml file to save data in readonly filed populated by onchange method.

This means the user cannot enter data in this field. When a field is read-only, it can only be viewed and cannot be modified.
name="versand_route_id" readonly="1"/>

If this field can change dynamically depending on another field and you want to save its content, you can use force_save="1".

​name="versand_route_id" readonly="1" force_save="1"/> 

I did it the following way, correct me if I am wrong, I forced saved the value. It works just fine. 

"<xpath expr="//field[@name='amount_total']" position="attributes">

<attribute name="force_save">True</attribute>

</xpath>"

Hi all,

The best solution is to add the attribute force_save="1" for the field in XML file.

We no need to install any third party module.

Hope it helps!

Hello

I hope below code will help you

In my case have Total year field is readonly and based on 'Date of birth' Total year will be update

Using onchange method, Can get Total year on field but when save that record Total Year field to set blank

Solution:-

Create new dummy field of total year and set that dummy field value on original field

Example:-

Python file

total_year = fields.Float()

total_year_copy = fields.Float()

from datetime import date​​

# Onchange method

@api.onchange('dob')
def onchange_dob(self):
today = date.today()
self.total_year = self.total_year_copy = today.year - dob.year - ((today.month, today.day) < (dob.month, dob.day))

# Create method

@api.model

def create(self, vals):

if 'total_year_copy' in vals:

vals.update({'total_year': vals.get('total_year_copy')})

return super(Project, self).create(vals)

# Write method

@api.multi

def write(self, vals):

if 'total_year_copy' in vals:

vals.update({'total_year': vals.get('total_year_copy')})

return super(Project, self).write(vals)

Xml File

<field name="total_year" readonly="1"/>

<field name="total_year_copy" invisible="1"/>

Hope this help you to save readonly records

Best Regards,

Ankit H Gandhi