nested form to update has-many parent AND create a *single* child object

Question

I want a form to edit a single new child object and the parent object in one go (in a has many relationship). If I create a new child in the controller with @child = @parent.children.new, the following works, but it displays input fields for all existing children.

<%= simple_form_for @parent do |p| %>
  <%= p.input :parent_attribute %>
  <%= p.simple_fields_for :children do |c| %>
    <%= c.input :child_attribute %>
  <% end %>
<% end %>

How can I display form input fields for only the single new child that was created?

Answer 1

If you want to display the new single child form within the parent form, you can do this:

<%= simple_form_for @parent do |p| %>
  <%= p.input :parent_attribute %>
  <%= p.simple_fields_for :child, @child do |c| %>
    <%= c.input :child_attribute %>
  <% end %>
<% end %>

Note that you have to specify both the child model name, as well as the child model object to simple_fields_for (or fields_for) to reference a specific model object.

If you only want to display the new single child form, you can do this:

<%= m.simple_form_for @child do |c| %>
  <%= c.input :child_attribute %>
<% end %>

This will create the form only for the new child record.