The alias_method test problem. How to best create an alias for an ActiveRecord association?
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Alias test

We have two very simple models in this application:

  1. Person

  2. Role

Whenever you manipulate a Person, you also assign that person a Role.

The problem

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.

The catch

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 %>

equates to

<%= 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)