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module ActiveRecord
module Locking
# == What is Optimistic Locking
#
# Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of
# conflicts with the data. It does this by checking whether another process has made changes to a record since
# it was opened, an ActiveRecord::StaleObjectError is thrown if that has occurred and the update is ignored.
#
# Check out ActiveRecord::Locking::Pessimistic for an alternative.
#
# == Usage
#
# 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 Optimistic
def self.included(base) #:nodoc:
base.extend ClassMethods
base.cattr_accessor :lock_optimistically, :instance_writer => false
base.lock_optimistically = true
base.alias_method_chain :update, :lock
base.alias_method_chain :attributes_from_column_definition, :lock
class << base
alias_method :locking_column=, :set_locking_column
end
end
def locking_enabled? #:nodoc:
self.class.locking_enabled?
end
private
def attributes_from_column_definition_with_lock
result = attributes_from_column_definition_without_lock
# If the locking column has no default value set,
# start the lock version at zero. Note we can't use
# locking_enabled? at this point as @attributes may
# not have been initialized yet
if lock_optimistically && result.include?(self.class.locking_column)
result[self.class.locking_column] ||= 0
end
return result
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)
begin
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_value(id)}
AND #{self.class.quoted_locking_column} = #{quote_value(previous_value)}
end_sql
unless affected_rows == 1
raise ActiveRecord::StaleObjectError, "Attempted to update a stale object"
end
affected_rows
# If something went wrong, revert the version.
rescue Exception
send(lock_col + '=', previous_value)
raise
end
end
module ClassMethods
DEFAULT_LOCKING_COLUMN = 'lock_version'
def self.extended(base)
class <<base
alias_method_chain :update_counters, :lock
end
end
# Is optimistic locking enabled for this table? Returns true if the
# #lock_optimistically flag is set to true (which it is, by default)
# and the table includes the #locking_column column (defaults to
# lock_version).
def locking_enabled?
lock_optimistically && columns_hash[locking_column]
end
# Set the column to use for optimistic locking. Defaults to lock_version.
def set_locking_column(value = nil, &block)
define_attr_method :locking_column, value, &block
value
end
# The version column used for optimistic locking. Defaults to lock_version.
def locking_column
reset_locking_column
end
# Quote the column name used for optimistic locking.
def quoted_locking_column
connection.quote_column_name(locking_column)
end
# Reset the column used for optimistic locking back to the lock_version default.
def reset_locking_column
set_locking_column DEFAULT_LOCKING_COLUMN
end
# make sure the lock version column gets updated when counters are
# updated.
def update_counters_with_lock(id, counters)
counters = counters.merge(locking_column => 1) if locking_enabled?
update_counters_without_lock(id, counters)
end
end
end
end
end
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