app/views/invoices/_form.html.erb

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<div>
  <%= form_for invoice do |f| %>
    <%= render "errors", object: invoice %>
    <div class="form-group">
      <%= f.label :invoice_number %>
      <%= f.number_field :invoice_number, class: "form-control" %>
    </div>
    <div class="form-group">
      <%= f.label :customer_id, "Customer" %>
      <%= f.select :customer_id, current_account.customers.collect{ |c| [c.first_name, c.id]},{},  class: "form-control" %>
    </div>
    <div>
      <%= f.submit "Submit", class: "btn btn-primary"%>
    </div>
  <% end %>
</div>