Question

I have the following code in a rails model:

foo = Food.find(...)
foo.with_lock do
  if bar = foo.bars.find_by_stuff(stuff)
    # do something with bar
  else
    bar = foo.bars.create!
    # do something with bar
  end
end

The goal is to make sure that a Bar of the type being created is not being created twice.

Testing with_lock works at the console confirms my expectations. However, in production, it seems that in either some or all cases the lock is not working as expected, and the redundant Bar is being attempted — so, the with_lock doesn’t (always?) result in the code waiting for its turn.

What could be happening here?

update
so sorry to everyone who was saying “locking foo won’t help you”!! my example initially didin’t have the bar lookup. this is fixed now.

Answer 1

A reason why a lock wouldn’t be working in a Rails app in query cache.

If you try to obtain an exclusive lock on the same row multiple times in a single request, query cached kicks in so subsequent locking queries never reach the DB itself.

The issue has been reported on Github.