tap/app/views/products/stock_entry.html.haml

.row
  #stock_entry.col-md-6.col-md-offset-3
    = form_for @product do |f|
      %table
        %tr
          %td Current stock
          %td= @product.stock
        %tr
          %td Purchase
          %td
            %input.form-control.new_stock{ type: :number, autofocus: true, data: { increment: true, target: "#product_stock" }}
        %tr
          %td New stock
          %td= f.number_field :stock, data:{ default: f.object.stock }, class: "form-control"
      = f.submit "Update stock", class: "btn btn-primary form-control"