here is something you don't see every day....

But given this completes modelling of saml, samlp and md SAML 2 xml schema lets do a close review of develop before we dump it into master
Actually, on further review down the page, this doesn't seem to be quite right. The idea is solid, just a minor implementation hiccup.

The error strings seem to be expecting `owners` to contain a list of the associations, but in fact it contains the associated objects. So when `owners` is empty, the error will read: `must be owned by  or '

Perhaps the best approach would be to take in a list of symbols and start like:

```ruby
def valid_owner(associations)
  owners = associations.map { |sym| send(sym) }
  # ...
end
```