#59 Optimistic Locking

Jul 18, 2007 | 7 minutes | Active Record

When two people attempt to update the same record near the same time, one of the updates will likely be overwritten. You can solve this problem with optimistic locking.

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There is a newer version of this episode, see the revised episode.

          migrations/011_add_products_lock_version.rb
        
add_column :products, :lock_version, :integer, :default => 0, :null => false

          products_controller.rb
        
def update
  @product = Product.find(params[:id])
  if @product.update_attributes(params[:product])
    flash[:notice] = "Successfully updated product."
    redirect_to product_path(@product)
  else
    render :action => 'edit'
  end
rescue ActiveRecord::StaleObjectError
  @product.reload
  render :action => 'conflict'
end

          _form.rhtml
        
<%= f.hidden_field :lock_version %>

          conflict.rhtml
        
<% title "Edit Product Conflict" %>

Someone edited the product the same time you did. Please re-apply your changes to the product.

<h2>Your Submission:</h2>
<pre>
<% params[:product].each do |name, value| %>
  <%=h name.humanize %>: <%=h value %>
<% end %>
</pre>

<h2>Edit Product:</h2>
<% form_for :product, :url => product_path(@product), :html => { :method => :put } do |f| %>
  <%= render :partial => 'form', :locals => { :f => f } %>
  <%= submit_tag 'Resolve' %>
<% end %>