when is a pessimistic lock released in rails?

Question

Assuming I'm doing something like this (from the Active Record Querying guide)

Item.transaction do  
  i = Item.first(:lock => true)  
  i.name = 'Jones'  
  i.save 
end 

Is the lock automatically released at the end of the transaction? I've looked at the Active Query guide and the ActiveRecord::Locking::Pessimistic docs, and couldn't find where it explicitly says where the lock is released.

Answer 1

I believe you'll want an "ensure" block to be certain the lock is released.

http://ruby-doc.org/core/classes/Mutex.src/M000916.html has:

  def synchronize
    lock
    begin
      yield
    ensure
      unlock
    end
  end

http://yehudakatz.com/2010/02/07/the-building-blocks-of-ruby/ seems to suggest, however, that the block structure of that method will automatically unlock.

Answer 2

Locking is not a function of rails, it is just adding the lock statement to the query, which will vary depending on the database that you are using. Pessimistic Locking takes a "pessimistic" view in thinking that every query is subject to corruption. So it is going to lock the database until you are finished with the query. so Lock > query > unlock. While these are fairly consistent database to database, it might be good to read up on the database documentation that you using for any database-specific things you should know.

Here is a good thread on optimistic vs. pessimistic locking that explains it better than I can. http://stackoverflow.com/questions/129329/optimistic-vs-pessimistic-locking