_orderitemform.html.rb

<% if orderitem.alreadyExists? %>
  <%= form_for [@order, orderitem] do |f| %>
	  <%= f.label :quantity %>
		<%= f.number_field :quantity, :min => 1, :value => alreadyexists.quantity.to_i %>
		<%= f.hidden_field :product_id, :value => orderitem.product.id %>
		<%= f.submit 'Change' %>
	<% end %>
<% else %>
	<% item = Orderitem.new(:order => @current_bom, :product => product, :quantity => 0) %>
	<%= form_for [@order, item] do |f| %>
	  <%= f.label :quantity %>
		<%= f.number_field :quantity, :min => 1 %>
		<%= f.hidden_field :product_id, :value => item.product.id %>
		<%= f.submit 'Add' %>
	<% end %>
<% end %>

database.txt

Before update_attribute:

+------------+-------------------+------------+
+  order_id  +  product_id       +  quantity  +
+------------+-------------------+------------+
+      1     +  C00070033000003  +    150     +
+------------+-------------------+------------+
+      1     +  C00080033000003  +     47     +
+------------+-------------------+------------+

After update_attribute:

+------------+-------------------+------------+
+  order_id  +  product_id       +  quantity  +
+------------+-------------------+------------+
+      1     +  C00070033000003  +    120     +
+------------+-------------------+------------+
+      1     +  C00080033000003  +    120     +
+------------+-------------------+------------+

orderitems_controller.rb

class OrderitemsController < ApplicationController
  def update
		orderitem = Orderitem.find_by_order_id_and_product_id(params[:order_id], params[:orderitem][:product_id])
		if orderitem
			if orderitem.update_attribute(:quantity, params[:orderitem][:quantity].to_i)
				flash[:notice] = "Saved!"
			else
				flash[:notice] = "Error updating your bill of materials"
			end
		else
			flash[:notice] = 'Error updating your bill of materials'
		end
		redirect_to(:back)
	end
end