# Implicit Transactions and after\_save callback

**URL:** <https://www.ruby-forum.com/t/implicit-transactions-and-after-save-callback/103657>\
**Category:** Rails\
**Created:** [May 8, 2007, 11:24pm UTC](https://www.ruby-forum.com/t/implicit-transactions-and-after-save-callback/103657 "2007-05-08T23:24:06Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![k9ert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/bbe5ce/32.png) [@k9ert](https://www.ruby-forum.com/u/k9ert)\
**Post date:** [May 8, 2007, 11:24pm UTC](https://www.ruby-forum.com/t/implicit-transactions-and-after-save-callback/103657/1 "2007-05-08T23:24:06Z")

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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  
transaction.

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?

## 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  
collisions.

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?

Kim

[http://rubyforge.org/projects/resman/](http://rubyforge.org/projects/resman/)

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<div class="post-metadata">

**Author:** ![k9ert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/bbe5ce/32.png) [@k9ert](https://www.ruby-forum.com/u/k9ert)\
**Post date:** [May 9, 2007, 11:40am UTC](https://www.ruby-forum.com/t/implicit-transactions-and-after-save-callback/103657/2 "2007-05-09T11:40:07Z")

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On 5/8/07, k9ert [removed\_email\_address@domain.invalid](mailto: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.

HTH,  
Isak

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<div class="post-metadata">

**Author:** ![k9ert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/bbe5ce/32.png) [@k9ert](https://www.ruby-forum.com/u/k9ert)\
**Post date:** [May 10, 2007, 7:43pm UTC](https://www.ruby-forum.com/t/implicit-transactions-and-after-save-callback/103657/3 "2007-05-10T19:43:10Z")

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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.

Kim