Caution when using before_destroy with model association
The other day, I was tasked with calling an external service when destroying a user. This kind of task is pretty straightforward in Rails, you call a method in a
before_destroy callback on your model.
This led me to a very instructive discovery about ActiveRecord callbacks and model dependencies.
First of all, here’s the very simplified schema of what we are working on:
As I was dealing with an external service, I wanted to process that task in a Sidekiq job. That job needed the user’s authorization uuid to complete its task.
So, part of my Rspec test was to expect the Sidekiq job to receive the user’s authorization uuid as parameter. But strangely enough, it was always failing.
I ran a
tail -f tmp/test.log to dig into execution logs:
As you can see the code tries to read the user’s
uuid after it was deleted.
Because on the top of the model you have :
has_many :authorizations, dependent: :delete_all
In fact when there is
dependent: delete_all or
They are implemented like
before_destroycallback, and since callbacks are executed in the order they’re defined, the
:dependentcallback is run before the
:checkcallback. The solution seems to be either force the
:dependentcallback to run last, or make it its own callback.
So at first I just inverted the order of
has_many (following “Ordering callbacks” guide)
It works, but you can do it differently. Use
prepend: true (see : ActiveSupport::Callbacks::ClassMethods#method-i-set_callback)
This is for us the cleaner way and this is what we chose.
What does prepend do?
prepend can be used for callbacks in general. It’s part of ActiveSupport.
:prepend— If true, the callback will be prepended to the existing chain rather than appended.
In our case, with the
usermodel we can see the callback order using
_destroy_callbacks method :
lambda in the callback chains are the
dependent: :delete_all or
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