Didn't work the code said by Mohammed Amal. It shows an internal server error.
from odoo import fields , models , api
class PurchaseOrderFields(models.Model):
_inherit = 'purchase.order'
_description = "Purchase Order"
#-------------------- -----------------------------------New Fields of PO----------- -----------------------------------------------
series_code = fields .Char( string = "Series_Code" )
series_desc = fields.Selection(([ "1" , "" ]) , related = 'series_code.series_desc' )
effective_date = fields.Date( string= "Effective Date" )
# customer_code = fields.Integer(string="Customer Code & Name")
# customer_desc = fields.Char(related='customer_code.customer_desc')
activity = fields.Selection([( "1" , "Choose one" ) , ( "2" , "Yes" ) , ( "3" , "No" )] ,
string = "Activity" )