Manually setting the index of a new element

If you have a field whose name attribute has to be manually written, you may want its index to be properly set by both Cocoon and Rails' form builder. Since Rails 4, you can do so by getting the index from the form builder.

<%= f.label :month %>
<%= select_month f.object[:month], {}, { name: "parent[childs_attributes][#{f.index}][month]" } %>

In this example we're using select_month, which only works as a standalone method, but can't be used with the form builder. Notice that we add f.index, which will add:

• new_childs when is a new record (added by Cocoon)
• the numeric index when it's a persisted record

This works because Cocoon itself replaces all instances of new_child when adding a new element, following Rails' naming conventions.