Forum: Ruby on Rails implicit Transactions and after_save callback

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k9ert (Guest)
on 2007-05-09 01:24
(Received via mailing list)
Hi group,

I have a Problem with ActiveRecord. I´m using SQL-Queries in the
after_save-Callbacks which assert, that the change in the db already
took place. But this is not the case, because we´re still in the

As far as I know, there is no callback which is called AFTER a commit
although this i think this would be very attractive. What can I do

Here is the Problem:
I have the class Event, Resources and ResourceUses. Think of them as
lessons (Events) and Teachers and Rooms (Resources).
class Event < ActiveRecord::Base
  has_many :resource_uses, :dependent => :destroy
  has_many :resources, :through => :resource_uses

  validates_presence_of :date
  validates_presence_of :from
  validates_presence_of :to

The Class "ResourceUse" has to check for Collisions based on the time-
data of the associated events. So in the after_save Callback, I run a
SQL-Query (in an instance-method of ResourceUse) to check for

Exactly: I have to do this for all ResourceUse-Instances which had
collisions with the event BEFORE the event saved.

How could this be solved, any hints?

Isak H. (Guest)
on 2007-05-09 13:40
(Received via mailing list)
On 5/8/07, k9ert <removed_email_address@domain.invalid> wrote:
> here?
>   validates_presence_of :from
> How could this be solved, any hints?

I think the simplest solution is a lock on the resource while
preparing your reservation, e.g.:

Whatever.transaction do

  # issue "select for update"; grab an exclusive lock on this row
  #    until end of transaction. any other tx trying to lock the same
  #    row will have to wait until it's released
  resource = Resource.find(..., :lock => true)

  # check for collisions and schedule ResouceUse if there's no conflict

end  # commit, freeing the lock on resource

Just be aware of deadlock issues; you don't want transactions to lock
multiple resources (if you must, make sure you always lock rows in the
same order -- but do try to find something more general to grab a
single lock on).

I think you could also set a transaction isolation level that lets you
see uncommitted writes, create your reservation and then check for
conflicts before commiting. Doubt this will work on all DBs.

k9ert (Guest)
on 2007-05-10 21:43
(Received via mailing list)
Thanks for clearing this up for me. I don´t like the idea do do it
with locks, so I´ll try to do it with the low isolation level in
combination with optimistic locking.

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