Odoo 11 - pull fields values from other models - odoo

Question

I have added a custom field (many2one) in crm.lead form and a custom class for that
Here is my code (I am using odoo 11):
models.py
from odoo import models, fields, api
# Add fields to Leads
class Leads_events(models.Model):
_inherit = 'crm.lead'
venue = fields.Many2one('crm.lead.venue',String="Venue")
venue_city = fields.Char(String="City")
venue_country = fields.Many2one('res.country', string="Country")
# Create venue class
class Leads_venue(models.Model):
_name = 'crm.lead.venue'
name = fields.Char(string="Venue name")
city = fields.Char(string="City")
country = fields.Many2one('res.country', string="Country")
address = fields.Text(string="Address")
website = fields.Char(string="Website")
phone = fields.Char(string="Phone")
fax = fields.Char(string="Fax")
notes = fields.Text(string="Notes")
views.xml
<record id="crm_lead_venue_form" model="ir.ui.view">
<field name="name">crm.lead.venue.form</field>
<field name="model">crm.lead.venue</field>
<field name="arch" type="xml">
<form string="Venues">
<group>
<group>
<field name="name"/>
<field name="country"/>
<field name="city"/>
<field name="address"/>
</group>
<group>
<field name="phone"/>
<field name="fax"/>
<field name="website"/>
</group>
<group>
<field name="notes" />
</group>
</group>
</form>
</field>
</record>
<record id="crm.crm_case_form_view_oppor_events" model="ir.ui.view">
<field name="name">crm_case_form_view_oppor_events</field>
<field name="model">crm.lead</field>
<field name="inherit_id" ref="crm.crm_case_form_view_oppor"/>
<field name="arch" type="xml">
<xpath expr="//notebook" position="before">
<group string="Venue">
<field name="venue"/>
<field name="event_city" string="City"/>
<field name="event_country"/>
</group>
</xpath>
</field>
</record>
This works fine. Now what I am trying to pull is the city and country from crm.lead.venue selected record to crm.lead 'venue_city' and 'venue_country' fields.
I looked at onchange function but I cannot find how to make it work...
I've tried this following another post I read but it is not working
#api.onchange('venue')
def onchange_venue(self):
if self.venue:
self.event_city = self.venue.city
self.event_country = self.venue.country
Is there anywhere I can find a documentation or help more complete on this ? The official documentation is not very precise on this matter.

Answer 1

The onchange method works when the user change the field in the view, and I cant see the field on your view, so the method would be never be called. You should add "venue" field to the form and test it again.
Edit
Have you seen that it is not event_city but venue_city? The onchange should be on leads_events. Did you test that onchange is executed and has no value? Or it doesnt even execute the function ?
#api.onchange('venue')
def onchange_venue(self):
if self.venue:
self.venue_city = self.venue.city
self.venue_country = self.venue.country