def function_name (self,cr,uid,contex=None):
     account_obj = self.pool.get('account.invoice')
     account_search = account_obj.search(cr,uid,[])
     for account in account_obj(cr,uid,account_search):
         print account.origin
# or

def funcion_name (self,cr,uid,context=None):
    account_obj = self.pool.get('account.invoice')
    account = account_obj.browse(cr,uid,id_account_invoice)
    print account.origin

I hope it helps you. regards

u are already inherit account.invoice, and why u create another field to get some field in same object?

u can rename the origin field and change with something that u need,

it can be different if that origin field will use in another object u can use self.pool.get and browse