We’ve run across the following concurrency issue with after-destroy:
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 after_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. It occurred to us that the
problem would not occur if the after_destroy callback was only called
when an object was actually deleted – i.e., the callback would only
be called if the number of rows affected by the destroy call is
greater than 0. Does this behavior sound reasonable? Should we propose
making this change to ActiveRecord? 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?
Michael