[Answer]-Django models Integrity error

You try to add non-saved Subscriber to Tour.

You need to call sub.save() before tour.subscribers.add(sub)

The tour must also be saved before adding sub.

The reason for this is the additional table that handles many to many relations (see docs). This table stores two Foreign keys – one for Tour and one for Subscribers. To save a row into this table both Tour and Subscriber should exist in database