Rails 4 yield additional form fields - ruby-on-rails

Question

I'm trying to DRY up one of my forms that sometimes has additional fields (for nested models) when making a new entry, but wouldn't need those fields when performing an update. As such, I was trying to pass the additional fields in via a block, but the form object isn't being passed properly.
Is there a way to pass a form object (or really any variable) into a yield?
Example code for reference:
_form.slim
= nested_form_for #model do |f|
.row
= f.label :name
= f.text_field :name, autofocus: true
...
= yield
...
= f.submit 'Save'
new.html.slim
== render layout: 'model/form' do
h3 Additional Fields
= f.fields_for :nested do |h|
= a.label :name, 'Nested Name'
= a.text_field :name
= a.link_to_remove do
= fa_icon 'times-circle-o'
= f.link_to_add "Add another nested model", :nested
edit.html.slim
== render layout: 'model/form'

Answer 1

To elaborate on my comment, this is how I'd do it using partials:
_form.slim
= nested_form_for #model do |f|
.row
= f.label :name
= f.text_field :name, autofocus: true
...
- if defined?(additional_fields)
h3 Additional Fields
= f.fields_for :nested do |h|
= a.label :name, 'Nested Name'
= a.text_field :name
= a.link_to_remove do
= fa_icon 'times-circle-o'
= f.link_to_add "Add another nested model", :nested
...
= f.submit 'Save'
new.html.slim
== render 'model/form', :additional_fields => true
edit.html.slim
== render 'model/form'
I might be missing something, but I'm not sure why this wouldn't work.