The current database schema of Foreman describes a one-to-many relation between Installation Medium and Operating Systems:

+-------+          +-----------------+
| Media |<*>----<1>| Operatingsystem |
+-------+          +-----------------+

ActiveRecord:
  Media :belongs_to Operatingsystem
  Operatingsystem :has_many Media

The implication of this design is, that the user has to create an installation medium for each operating system created in Foreman. This makes sense for installation media which contain a certain the version number or architecture of the operating system in their URI or path, like the ones of Fedora or CentOS Linux.

With Debian-based Linux distributions on the other hand, one mirror can serve several versions of the operating system from the same URI/path. This results in several identical installation media.

For example, having the operating systems "Ubuntu 8.04 LTS", "Ubuntu 9.10", "Ubuntu 10.04 LTS" each in "x86" and "x86_64" variants need 6 identical installation media in Foreman while one mirror would be sufficient.

By creating a many-to-many relation between Installation Media and Operating Systems, the user could "reuse" an installation medium for several operating system.

+-------+          +-----------------+
| Media |<*>----<*>| Operatingsystem |
+-------+          +-----------------+

ActiveRecord:
  Media has_and_belongs_to_many Operatingsystem
  Operatingsystem has_and_belongs_to_many Media

PS: Assigned to Paul Kelly due to recommendation of Ohad Levy.