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module ActiveRecord
# Active Records support optimistic locking if the field <tt>lock_version</tt> is present. Each update to the
# record increments the lock_version column and the locking facilities ensure that records instantiated twice
# will let the last one saved raise a StaleObjectError if the first was also updated. Example:
#
# p1 = Person.find(1)
# p2 = Person.find(1)
#
# p1.first_name = "Michael"
# p1.save
#
# p2.first_name = "should fail"
# p2.save # Raises a ActiveRecord::StaleObjectError
#
# You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging,
# or otherwise apply the business logic needed to resolve the conflict.
#
# You must ensure that your database schema defaults the lock_version column to 0.
#
# This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
# To override the name of the lock_version column, invoke the <tt>set_locking_column</tt> method.
# This method uses the same syntax as <tt>set_table_name</tt>
module Locking
def self.included(base) #:nodoc:
base.class_eval do
alias_method_chain :update, :lock
end
end
def update_with_lock #:nodoc:
return update_without_lock unless locking_enabled?
lock_col = self.class.locking_column
previous_value = send(lock_col)
send(lock_col + '=', previous_value + 1)
affected_rows = connection.update(<<-end_sql, "#{self.class.name} Update with optimistic locking")
UPDATE #{self.class.table_name}
SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))}
WHERE #{self.class.primary_key} = #{quote(id)}
AND #{lock_col} = #{quote(previous_value)}
end_sql
unless affected_rows == 1
raise ActiveRecord::StaleObjectError, "Attempted to update a stale object"
end
return true
end
end
class Base
@@lock_optimistically = true
cattr_accessor :lock_optimistically
def locking_enabled? #:nodoc:
lock_optimistically && respond_to?(self.class.locking_column)
end
class << self
def set_locking_column(value = nil, &block)
define_attr_method :locking_column, value, &block
end
def locking_column #:nodoc:
reset_locking_column
end
def reset_locking_column #:nodoc:
default = 'lock_version'
set_locking_column(default)
default
end
end
end
end
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