Forum: Ruby on Rails DB strategy - Lock Table?

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Shagy M. (Guest)
on 2008-10-17 22:18
Hi all,

I have a process which creates a new object from a backgroundrb job.
This object is versioned and the version is based on the existing
objects in this table.  For example, I've got a table called
spreadsheets and it has a column called version.  Each version is
incremental and when a new one is created, the last version is found by
a query.

The problem I'm having is that when two background jobs are creating the
same type of object, the read the table at the same time and thus assign
the same version number to the object they are saving.  I end up with
two objects with the same version number.  I cannot make it a unique
column because it the table holds different objects with different
version paths.

Would locking the entire table be a good solution to this problem? It
seems like it will cause timeout problems. If this is the way to go,
what is the best "rails" way to accomplish this?  If not, what
suggestions do you have on how to solve this problem?

Thanks much,

Shagy
Frederick C. (Guest)
on 2008-10-17 23:40
(Received via mailing list)
On 17 Oct 2008, at 19:18, Shagy M. <removed_email_address@domain.invalid>
wrote:

>
> The problem I'm having is that when two background jobs are creating
> the
> same type of object, the read the table at the same time and thus
> assign
> the same version number to the object they are saving.  I end up with
> two objects with the same version number.  I cannot make it a unique
> column because it the table holds different objects with different
> version paths.

Just create a unique index on the pair (or triple etc. ) of columns
that you are  interested in.

Fred
Shagy M. (Guest)
on 2008-10-18 00:30
I think I've solved it by using :lock in the find like this:

spreadsheets = Spreadsheet.find(:all,
                                :conditions => ["parent_id = ?",
self.id],
                                :lock => "FOR UPDATE")

This tables uses acts_as_tree and originally I was just calling
self.children.
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