Main purpose is to prevent orphaned groups.
It's not possible to use the ON DELETE CASCADE trigger here, to remove
the group together with the last feed that is assigned to this group.
The ON DELETE CASCADE trigger will raise an foreign key violation error
in cases where the removed feed is not the last feed associated to a
group.
The purge_groups() call has been moved to the remove group functions,
since it's the only way to create an orphaned group.
- each feed can have multiple groups assigned
- group assignments are done on the add or edit feed page
- groups are only visible on the unread page
- groups are exported via the fever api
- it's not possible do delete a group manually from the database (the group will be removed automatically, as soon as the last association of a group to a feed is removed)
- if you try to create an already existing group, the existing group is used to prevent duplicates
