vendor/rails/activerecord/lib/active_record/locking/pessimistic:
# Pass <tt>:lock => true</tt> to ActiveRecord::Base.find to obtain
an exclusive
# lock on the selected rows:
# # select * from accounts where id=1 for update
# Account.find(1, :lock => true)
#
# Pass <tt>:lock => 'some locking clause'</tt> to give a database-
specific locking clause
# of your own such as 'LOCK IN SHARE MODE' or 'FOR UPDATE NOWAIT'.
#
# Example:
# Account.transaction do
# # select * from accounts where name = 'shugo' limit 1 for
update
# shugo = Account.find(:first, :conditions => "name =
'shugo'", :lock => true)
# yuko = Account.find(:first, :conditions => "name =
'yuko'", :lock => true)
# shugo.balance -= 100
# shugo.save!
# yuko.balance += 100
# yuko.save!
# end
#
# You can also use ActiveRecord::Base#lock! method to lock one
record by id.
# This may be better if you don't need to lock every row. Example:
# Account.transaction do
# # select * from accounts where ...
# accounts = Account.find(:all, :conditions => ...)
# account1 = accounts.detect { |account| ... }
# account2 = accounts.detect { |account| ... }
# # select * from accounts where id=? for update
# account1.lock!
# account2.lock!
# account1.balance -= 100
# account1.save!
# account2.balance += 100
# account2.save!
# end
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