Updating ForeignKey with non-existing reference breaks functionality.

[MiguelSR]

When you try to update some element's foreign key providing an invalid value (i.e. not existing in the related table), it's stored anyway in DB and prevent you to do further operations on that element.

Example:

# Valid request

/usr/src/app # python -m my_module config url update 1 --force --client 1
Changed 1 records.

client  <Client id=1>
id      1
path    'foo'

# Bad request

/usr/src/app # python -m my_module config url update 1 --force --client -1
[...]
bugloader.models.ClientDoesNotExist: Instance matching query does not exist:
SQL: SELECT "t1"."id", "t1"."name" FROM "client" AS t1 WHERE ("t1"."id" = ?)
PARAMS: [-1]

From this moment every single operation gives an error, unless you pass the "-f" option to update the field or delete the row.