module ActiveRecord::Locking::Pessimistic
Pessimistic Locking
Locking::Pessimistic
provides support for row-level locking using SELECT … FOR UPDATE and other lock types.
Chain ActiveRecord::Base#find
to ActiveRecord::QueryMethods#lock
to obtain an exclusive lock on the selected rows:
# select * from accounts where id=1 for update
Account.lock.find(1)
Call lock('some locking clause')
to use 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 nowait
shugo = Account.lock("FOR UPDATE NOWAIT").find_by(name: "shugo")
yuko = Account.lock("FOR UPDATE NOWAIT").find_by(name: "yuko")
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.where(...)
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
You can start a transaction and acquire the lock in one go by calling with_lock
with a block. The block is called from within a transaction, the object is already locked. Example:
account = Account.first
account.with_lock do
# This block is called within a transaction,
# account is already locked.
account.balance -= 100
account.save!
end
Database-specific information on row locking:
Public instance methods
Obtain a row lock on this record. Reloads the record to obtain the requested lock. Pass an SQL locking clause to append the end of the SELECT statement or pass true for “FOR UPDATE” (the default, an exclusive row lock). Returns the locked record.
Source code GitHub
# File activerecord/lib/active_record/locking/pessimistic.rb, line 69
def lock!(lock = true)
if persisted?
if has_changes_to_save?
raise(<<-MSG.squish)
Locking a record with unpersisted changes is not supported. Use
`save` to persist the changes, or `reload` to discard them
explicitly.
Changed attributes: #{changed.map(&:inspect).join(', ')}.
MSG
end
reload(lock: lock)
end
self
end
Wraps the passed block in a transaction, reloading the object with a lock before yielding. You can pass the SQL locking clause as an optional argument (see lock!
).
You can also pass options like requires_new:
, isolation:
, and joinable:
to the wrapping transaction (see ActiveRecord::ConnectionAdapters::DatabaseStatements#transaction
).
Source code GitHub
# File activerecord/lib/active_record/locking/pessimistic.rb, line 92
def with_lock(*args)
transaction_opts = args.extract_options!
lock = args.present? ? args.first : true
transaction(**transaction_opts) do
lock!(lock)
yield
end
end