Form as a Wrapper

The form_for helper method sets up the entire form, creating the HTML form element but also providing context for all of the fields in the form. The form_for method is a bit sneaky, too. Both the new.html.erb view and the edit.html.erb view use form_for the same way:

<%= form_for(@person) do |f| %>
...
<% end %>

However, the generated form element looks very different, depending on what exactly is in @person. If @person is just an empty object structure, form_for will work on the assumption that this is to create a new object. If @person actually contains data, however, form_for will assume that its form is editing that object and create a different-looking form element, plus a hidden field to enable Rails’ REST capabilities.