I'm trying to decide how best to set up (if at all) foreign key constraints for my rails application. I have a model Response
that belongs_to
a Prompt
. I would like to use :dependent => :destroy
to have destroy called on every Response
that belongs to a deleted Prompt
and I'm trying to decide what delete constraint I should place on my foreign key.
In short I want advice about how I can get best take advantage of both the destroy method on dependent objects and foreign key constraints to ensure cruft doesn't accumulate and reflect the logical structure of the data being stored. Several earlier questions such as Should I use ON DELETE CASCADE, :dependent => :destroy, or both? and Rails: delete cascade vs dependent destroy asked which was better but they don't really say much about how the two choices interact and in what order they are triggered or seemed vague on the point.
As I see it the considerations seem to break up into a few pieces:
- Does
:dependent => :destroy
call destroy first on the dependent objects before removing the parent from the database so destroy will still be called on these objects even if I use cascade delete? Does
:dependent => :destroy
remove the dependent objects from the database before (or in a transaction with) removing the parent from the database? In other words if I set cascade to nullify will the database end up wastefully nullifying the references on the child objects before they are deleted?Are deletes issued as a result of the original destroy and chained
:dependent => :destroy
options wrapped in a transaction or will unfortunately timed crashes leave cruft in the database if I don't set cascade delete?- Finally will
:dependent => :destroy
ensure the parent object is deleted from the database if I use restrict as the foreign key on_delete option?
With dependent: :destroy
in a transaction rails first destroys all dependencies, and only then deletes the record itself.
There may be a race condition: if a dependent record was added just after rails read collection for destroying, but not deleted parent yet - it may be left over. Let's call these "race condition records" below.
yes, you can use
dependent: :destroy
andon delete cascade
, this way some children (race condition ones) can be deleted without callbacks. If callbacks are mandatory -on delete restrict
together with some locking and explicit children deletion may be better. This is somewhat likevalidates :some_field, uniqueness: true
that is better to be backed by unique index, only database itself can ensure data consistency.since parent is deleted last,
on delete nullify
will not get in the way (you'll get nullified race condition records)there's transaction wrapping all deletes, only race condition records can be left over
on delete restrict
overdependent: :destroy
will trigger only for race condition records (and roll back whole transaction), but if there was no race condition - rails will happily delete everything.
来源:https://stackoverflow.com/questions/35413592/what-does-rails-do-with-both-dependent-destroy-and-cascade-delete-nullify-r