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== lock



=== Purpose


The `lock` method obtains a pessimistic lock using an SQL `select ... for update`.


=== Examples


[source,groovy]
----
def book = Book.get(1)
book.lock()
----


=== Description


The `lock` method obtains a pessimistic lock on an instance, locking the row in the database with `select ... for update`. The `lock` method is equivalent to using Hibernate's https://javadoc.io/doc/org.hibernate/hibernate-core/{hibernate5Version}/org/hibernate/LockMode.html#UPGRADE[LockMode.UPGRADE] in combination with the https://javadoc.io/doc/org.hibernate/hibernate-core/{hibernate5Version}/org/hibernate/Session.html#lock(java/lang/Object, org/hibernate/LockMode)[lock] method.

The lock is automatically released when the transaction commits. In Grails this is typically after an action has finished executing.

Refer to the section on  link:{hibernate5GuideFromRef}index.html#locking[Optimistic and Pessimistic locking] in the user guide for info.
