Use AR relations to save fields/inputs and values

Instead of saving/destroying fields/inputs and values one at time
Use the Active Record relations to save these with a single save
call on the parent. This is done by adding the :autosave to the has_many
relations

It also includes the removal of to_i calls on the id fields. This will
help us when we switch to GIT and have strings as id's and the to_i
calls would fail
This is technically different because it will raise an exception when not found whereas the original just returned an empty array.  Prefer `find_by_id` instead.