_form.html.erb (wallet new view)

_expense_fields.html.erb

<%= f.input :name, label: 'Expense name', class: 'control-label' %>
<%= f.input :amount, as: :currency, wrapper: 'prepend' %>

gistfile1.rb

<%= simple_form_for @wallet, url: create_budget_path, html: {class: "form-horizontal"} do |f| %>
  <div class="control-group">
    <%= f.input :name, label: 'Budget name', class: 'control-label' %>
    <%= f.input :amount, wrapper: :prepend, wrapper_html: {class: 'budget-amount'} do %>
      <%= f.input_field :amount, as: :currency %>
    <% end %>
    <div id="plan-budget">
	    <div id="plan-budget-expenses">
	      <%= f.simple_fields_for :expenses do |builder| %>
	        <%= render 'expenses/expense_fields', f: builder %>
	      <% end %>
	    </div>
      <%= link_to '+', '#', class: 'btn'%>
    </div>

    <div class="form-actions">
      <%= f.button :submit, 'Add budget', class: 'btn btn-primary' %>
      <%= link_to 'Plan this budget', '#', class: 'btn btn-success'%>
    </div>
  </div>
<% end %>