Odoo - Dynamic Domain of a Many2One field

If you have a requirement to restrict the selection of a Many2one field, eg, customers or vendors, based on the value of another field or other complex condition, you may use the following 2 example codes.

opportunity_id = fields.Many2one('crm.lead', string="Opportunity")
 customer_name = fields.Many2one('res.partner', string='Customer')

@api.onchange('customer_name')
 def onchange_customer_name(self):
  for rec in self:
  return {'domain': {'opportunity_id': [('partner_id', '=', rec.customer_name.id)]}}

2. This is more complex way to retrieve the drop down value with logic. You will need a domain field (many2many) that will return the list of the vendors.

@api.multi
 @api.depends("place_of_delivery")
 def _get_vendor_domain(self):
  for obj in self:
  vendor_ids = []
  for line in obj.place_of_delivery.haulage_charge_line_ids:
  for vendor_id in line.vendor_id.ids:
  vendor_ids.append(vendor_id)
  self.vendor_list = [(6,0, vendor_ids)]
 
 vendor = fields.Many2one('res.partner', string="Vendor")
 vendor_list = fields.Many2many('res.partner',store=True,compute=_get_vendor_domain)

<field name="vendor_list" invisible="1"/>
 <field name="vendor" domain="[('id','in',vendor_list)]"/>

Note that there is a web domain field to do that as well.