This app demonstrates what appears to be a defect in the handling of a chain of has_one, through relationships when a before_save callback is present. This only occurs if the has_one, through chain has at least 2 intermediate classes.

The classes used here are: King, Queen, Jack and Ten.

King  has_one Queen.
Queen belongs_to King  and has_one Jack.
Jack  belongs_to Queen and has_one Ten.
Ten   belongs_to Jack  and has_one Queen through Jack and has_one King through Queen

Then with a before_save callback on Ten (Ten#refer_to_king), a reference to king is nil during the save process. This appears to be a defect. Note direct references to jack and queen are okay from the callback.

Running tests will result in output from the callback that demonstrates the problem (reference to king from ten in call is nil).