app/views/invoices/_form.html.erb
<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>