Cannot use foreign keys between tables of the same non-default db

[muraveill]

When one has multiple databases, one can use the using keyword (or a router class) to query or save entries to a specific db.
Cross-database references (foreign keys) are not supported, but nothing should prevent one from using references between tables of the _same_ db. But it does not work:

person = People(firstname=firstname, lastname=lastname)
person.save(using=MY_DB)

newuser = User(username=username, person=person)
newuser.save(using=MY_DB) # to illustrate, but it fails at the previous line

returns

Cannot assign "<People: People object>": "People" instance isn't saved in the database.

Using a router class telling models User and People to always use MY_DB, instead of the 'using' keyword, does not help.
At least there is a point missing in the documentation.