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GitHub User: mattlondon
You can accomplish this on the server side by building the nested object in your view. Use the "build_(association name)" method, which is provided by Active Record.
Inside the _action_fields.html.erb partial:
<%= f.label :step_height, "Step Height (in feet)" %>
<%= f.text_field :step_height %>
<% f.object.build_action %> **HERE**
<%= f.fields_for :action do |builder| %>
<%= render 'action_fields', f: builder %>
<% end %>
Now the fields_for :action block will execute and render the action_fields partial.
Naturally you would want to use this in the sub-nested partials as well.