Model lock

You can use lock using with_lock method on query collection:

Clear::SQL.transaction do
# SELECT * FROM users WHERE organization = 'Crystal Lang' FOR UPDATE
User.where(organization: "Crystal Lang").with_lock.each do |user|
# Do something with your users

with_lock offers optional parameters (default: "FOR UPDATE"), to setup the lock options you want (ex: with_lock("FOR UPDATE SKIP LOCKED"))

See PostgreSQL deep explanation about locking here.

Lock work only inside transaction. Without transaction block, the call might fail.