I've noticed a couple of people tripping up (myself included) on using
the new nested model forms across has_one relationships. Because
@parentModel.childModel returns nil (as opposed to  for a has_many
relationship) it is not possible to assign attributes to a new child
model in a has_one relationship without running
I've explained the exact error and the solution in more detail here :
Would it be possible to amend the rails guides and the documentation
on the rails blog on nested model forms to clarify this extra step
which is required for has_one relationships? The documentation as it
stands gives the impression that has_many and has_one are
interchangeable without any modification to your nested form code.
Just a simple note on this extra build_ step could save users a lot of
P.S The complex_form_examples were invaluable in helping me track down
where I was going wrong in this instance. These are a great idea for
when you are having problems getting your own code to work and you
need a working example. Thank you to those responsible!