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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 <tt>ActiveRecord::StaleObjectError</tt> exception is thrown if that has occurred
# and the update is ignored.
#
# Check out <tt>ActiveRecord::Locking::Pessimistic</tt> for an alternative.
#
# == Usage
#
# Active Records support optimistic locking if the field +lock_version+ 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
#
# Optimistic locking will also check for stale data when objects are destroyed. Example:
#
# p1 = Person.find(1)
# p2 = Person.find(1)
#
# p1.first_name = "Michael"
# p1.save
#
# p2.destroy # 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.
#
# This locking mechanism will function inside a single Ruby process. To make it work across all
# web requests, the recommended approach is to add +lock_version+ as a hidden field to your form.
#
# 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
extend ActiveSupport::Concern
included do
cattr_accessor :lock_optimistically, :instance_writer => false
self.lock_optimistically = true
end
def locking_enabled? #:nodoc:
self.class.locking_enabled?
end
private
def increment_lock
lock_col = self.class.locking_column
previous_lock_value = send(lock_col).to_i
send(lock_col + '=', previous_lock_value + 1)
end
def update(attribute_names = @attributes.keys) #:nodoc:
return super unless locking_enabled?
return 0 if attribute_names.empty?
lock_col = self.class.locking_column
previous_lock_value = send(lock_col).to_i
increment_lock
attribute_names += [lock_col]
attribute_names.uniq!
begin
relation = self.class.unscoped
stmt = relation.where(
relation.table[self.class.primary_key].eq(id).and(
relation.table[lock_col].eq(quote_value(previous_lock_value, self.class.columns_hash[lock_col]))
)
).arel.compile_update(arel_attributes_values(false, false, attribute_names))
affected_rows = connection.update stmt
unless affected_rows == 1
raise ActiveRecord::StaleObjectError.new(self, "update")
end
affected_rows
# If something went wrong, revert the version.
rescue Exception
send(lock_col + '=', previous_lock_value)
raise
end
end
def destroy #:nodoc:
return super unless locking_enabled?
destroy_associations
if persisted?
table = self.class.arel_table
lock_col = self.class.locking_column
predicate = table[self.class.primary_key].eq(id).
and(table[lock_col].eq(send(lock_col).to_i))
affected_rows = self.class.unscoped.where(predicate).delete_all
unless affected_rows == 1
raise ActiveRecord::StaleObjectError.new(self, "destroy")
end
end
@destroyed = true
freeze
end
module ClassMethods
DEFAULT_LOCKING_COLUMN = 'lock_version'
# 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
def locking_column=(value)
@original_locking_column = @locking_column if defined?(@locking_column)
@locking_column = value.to_s
end
# Set the column to use for optimistic locking. Defaults to +lock_version+.
def set_locking_column(value = nil, &block)
deprecated_property_setter :locking_column, value, block
end
# The version column used for optimistic locking. Defaults to +lock_version+.
def locking_column
reset_locking_column unless defined?(@locking_column)
@locking_column
end
def original_locking_column #:nodoc:
deprecated_original_property_getter :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
self.locking_column = DEFAULT_LOCKING_COLUMN
end
# Make sure the lock version column gets updated when counters are
# updated.
def update_counters(id, counters)
counters = counters.merge(locking_column => 1) if locking_enabled?
super
end
# If the locking column has no default value set,
# start the lock version at zero. Note we can't use
# <tt>locking_enabled?</tt> at this point as
# <tt>@attributes</tt> may not have been initialized yet.
def initialize_attributes(attributes, options = {}) #:nodoc:
if attributes.key?(locking_column) && lock_optimistically
attributes[locking_column] ||= 0
end
attributes
end
end
end
end
end
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