[Answer]-Cannot save new object in one to one relationship using django rest framework

You should call .save() over idSerial first, it will returns eventID updated instance, then you can set number attribute then save it.

 idSerial = EventIdS(event, data=db_data)
 if idSerial.is_valid():

    eventID = idSerial.save()
    eventID.number = event_id
    eventID.save()