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module ActiveRecord
module Locking
# 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>.
def self.append_features(base)
super
base.class_eval do
alias_method :update_without_lock, :update
alias_method :update, :update_with_lock
end
end
def update_with_lock
if locking_enabled?
previous_value = self.lock_version
self.lock_version = previous_value + 1
affected_rows = connection.update(
"UPDATE #{self.class.table_name} "+
"SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))} " +
"WHERE #{self.class.primary_key} = #{quote(id)} AND lock_version = #{quote(previous_value)}",
"#{self.class.name} Update with optimistic locking"
)
raise(ActiveRecord::StaleObjectError, "Attempted to update a stale object") unless affected_rows == 1
else
update_without_lock
end
end
end
class Base
@@lock_optimistically = true
cattr_accessor :lock_optimistically
def locking_enabled?
lock_optimistically && respond_to?(:lock_version)
end
end
end
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