Model with method_missing - rails 3, ruby 1.9.2 raises exception

Say you have two models Post and Author.

And Post has_many :authors

Finally, method_missing is overridden in Author.

Then if you do

Post.limit(1).first << Author.limit(1).first

You get an exception Author does not define :to_ary

Or has there been a change in rails 3 that I have completely
overlooked while migrating my app?