Optimistic locking is used to restrict updation of a record by multiple user at the same time . For example we have two user u1 and u2 and both of them is trying to edit the same record from a model City . While u1 is editing the record u2 comes and update the record . Now if u1 submits the form he will get ActiveRecord::StaleObjectError exception .

@city1 = City.find(42) #accessed by u1
@city2 = City.find(42)    #accessed by u2

@city2.zip_code = 248001
@city2.save

@city1.zip_code = 700115
@city1.save #raises ActiveRecord::StaleObjectError

Activerecord does this by maintaining a column lock_version. The default value of this column should be 0 . In fact we can name the column as custom_locking_column and then go back to model to set this as locking column.

  class City < ActiveRecord::Base
  set_locking_column :custom_locking_column
end

Optimistic locking prevents the changes made by one user to be overwritten by another user unknowingly.

Here one thing we need to take care we must have the locking column in the edit form like this

<%= form_for @city do |form| %>
   <%= form.hidden_field :lock_version %>
   <%# ... other inputs %>
<% end %>

If we want to turn off this behaviour we can do it like this:

ActiveRecord::Base.lock_optimistically = false

in the application.rb file.