Rails core - Jun 2008 - counter_cache concurrency issues; seeking feedback on potential solution

We''ve run across the following concurrency issue with the
has_many :counter_cache option:

We have an Post model that has_many Recommendations and uses a
counter_cache:

class Post < ActiveRecord::Base
  has_many :recommendations, :counter_cache => true
end

And, in a controller, we have a delete_recommendation action:

def delete_recommendation
  r = Recommendation.find(params[:id])
  r.destroy
end

Most of the time, the recommendation counter is updated properly. But
if many delete_recommendation requests are processed at once for the
same recommendation object, then our counts can get out of sync. The
problem is that, in many of the simultaneous requests, the
Recommendation.find(params[:id]) line successfully retrieves the
object, and is therefore able to call destroy on it. Obviously only
one of the resultant "DELETE FROM" SQL calls will actually remove any
rows from the database, but the before_destroy callback will be called
for every request regardless, which means the count will be improperly
decremented multiple times.

There are many ways to work around this problem, but this particular
pattern is common, and it seems like the Rails counter_cache
implementation ought handle this case. One possible solution:

* Move the counter_cache decrement from the before_destroy callback to
the
after_destroy callback
* Only call the after_destroy callback if the call to destroy actually
deleted rows from the database (i.e., rows affected > 0)

Does this behavior sound reasonable? Are there any scenarios where you
would want the after_destroy callback to be called even if destroy did
not delete anything from the database? Or is there a better solution?

Michael
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