Bug 28490: Revert "Bug 26995: Remove references to relationship column"

Bug 28490: Revert "Bug 26995: Remove references to relationship column"

This reverts commit 1cc8a77c287666ed878ea58f17216793f52ce224.

It was part of a patch set that removed actively used data and must be
therefore reverted.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Petro Vashchuk <stalkernoid@gmail.com>
Signed-off-by: Emmi Takkinen <emmi.takkinen@koha-suomi.fi>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>