We have two very simple models in this application:
Whenever you manipulate a Person, you also assign that person a Role.
Suppose we wanted to change the attribute name of a Person's role to responsibility. Suppose also that there is insufficient test coverage to find all the places where that attribute is used. So that we don't break existing pages, we want a way to create a Person#responsibility attribute such that if one of either Person#role or Person#responsibility is manipulated, the other will reflect the changes immediately.
The attempted solution
Person belongs_to a Role. Person therefore has a database column role_id.
Our naive solution will be to write two alias_method definitions for the getter and setter of Person#role.
There is no Person#responsibility_id.
Adding formtastic to the mix
Formtastic does some interesting stuff behind-the-scenes to figure out that
<%= f.input :role %>
<%= f.label :role_id %> <%= f.collection_select :role_id, :id, :title %>
This works fine in the PersonController#new view, because we're referencing :role.
It doesn't work on the PersonController#edit view. For some reason, formtastic guesses that :responsibility is a String value.
Thus, when we attempt to update the Person, we get the error:
Role(#2174572860) expected, got String(#2148412400)