This is an example of implementing dynamic fields of has_many associations.
We have two models Price and Product with has_many association as defined below.

price.rb

class Price < ActiveRecord::Base
  belongs_to :product
end

product.rb

class Product < ActiveRecord::Base
  has_many :prices
  accepts_nested_attributes_for :prices, allow_destroy: true

  def initialize(attrs=nil)
    super
    self.prices << Price.new if self.prices.empty?
  end
end

We define route for adding dynamic price fields.

routes.rb

    resources :products do
      get :new_price, on: :collection
    end

In controller, we make sure to include prices_attributes in permitted attributes.
We don't need any controller action for new_price since it will be processed directly by new_price.js.erb

products_controller.rb

  private
  def product_params
    params.require(:product).permit(:name, :other_product_fields...
                                     prices_attributes: [
                                       :id, :value, ...,
                                     ])
  end

This is the javascript for rendering new dynamic fields based on the next_index received.
We render the dynamic fields with child_index set as next_index. Also, next_index of new button is incremented.

new_price.js.erb

<% field = "" %>
<% form = "" %>

<%# build a single price field %>
<% form_for Product.new do |f| %>
  <% form = f %>
  <% f.fields_for :prices, Price.new, child_index: params[:next_index] do |fa,idx| %>
    <% field = fa %>
  <% end %>
<% end %>
$("#new-price").append("<%= escape_javascript(render(partial: "price", locals: {f: form, fa: field}))%>");
$("#new-price-btn").attr({href: "<%= escape_javascript(new_price_products_path(next_index: params[:next_index].to_i+1)) %>"})

In price partial file, _destroy checkbox is included so that we can remove dynamic fields on save.

_price.html.haml

/ Price fields ...

/ Actions
= fa.check_box :_destroy

Finally, we add new price button in product form.

_form.html.haml

=form_for @product do |f|
  / Product fields...
  =f.fields_for :prices do |fa,idx|
    / Price 
    = render partial: "price", locals: {f: f, fa: fa}
  / Add new price button
  = link_to new_price_products_path(next_index: f.object.prices.length), id: "new-price-btn", class: "btn btn-info btn-sm", remote: true do
    =t("helpers.links.new")