Database which allows object to be "child" of any other object

I would like to cut down on the number of strictly-defined complex object types, instead forming them from a composition of attached atomic metadata objects. A user could have an attached phone number, an attached address, an attached photo; but furthermore, an address could have an attached photo or phone number.

This means that in the DB, my relationship cannot be simply from the "PhoneNumber" table to the "User" table. It must allow relationships from each metadata table to, potentially, every other metadata table. This **could** be done using n^2 association tables for each parent/ child combination of metadata tables, but that would suck major donkey balls. It could also be done by having a generalized association table which specifies both the table *and* the key of the parent and child in the relationship, but I can think of no way to set up automatic referential integrity checking when doing this.

My questions are:

• Is this a viable approach? Has it been done before, is there a common name for it?
• Is the "generalized association table" idea compatible with automatic referential integrity checking? If so, how?
• Are there any systems out there implemented in this way that I could examine for research, open-source or not?