A casebook can be added to two series, leading to an inconsistent state

[lizadaly]

Because the CommonTitle model has both a direct foreign key field for the current title, and an indirect relationship through the Casebook model to a common_title id, it's theoretically possible for a casebook to end up in two series as the current title in one but a subsidiary title in another.

And indeed it's possible to do this through the UI:

• Have two series with at least 2 books in each.
• In the manage screen, select the current title from one series, and then select "Add selected to group" in the other series.

You'll get a dashboard like this:

with the same casebook in two series, under different fields.