Relationships as first-class citizens

Here is a link to a research project for having relationships as first-class citizens in Java:

http://www.cl.cam.ac.uk/%7Eaw345/talks/ecoop05.pdf

Not a new idea, the article mentions another dating back from 1987:

• J. Rumbaugh. Relations as semantic constructs in object-oriented development. (OOPSLA 1987)
  

This is for sure a topic that will make its way to a programming language someday. However, the above article indicates that this is a tough subject and that many of consequences of having relationships as first-class citizens are not yet mastered: inheritance, multiplicities, construction,...

I would bet that adding this kind of functionality via a mixin in Ruby could be a great way of getting the details right, in the manner of what is been done for ActiveRecords in RubyOnRails.