Use foreign_key to reference parent association

[slucaskim]

After a db update happens on a child record, identity_cache has mechanisms to invalidate the cache on associated parent records that have the child record as an embedded cache association (ex. cache_has_many :children, embed: true). This works even for "old" parent records where the child incurs an update where it no longer belongs to the old parent - identity cache is designed to invalidate the cache on the old parent record as well.

To know which parent records to invalidate the cache for, identity_cache first understands which attributes on the child records changed, then detects which of those changes are for parent associations. The second step uses the Rails association_foreign_key attribute. This works in most cases, but there is a bug when it comes to associations defined with custom foreign keys. Take the following example:

class Parent < ActiveRecord::Base
  has_many :children, foreign_key: :person_id
end

class Child < ActiveRecord::Base
  belongs_to :parent, foreign_key: :person_id
end

Child.reflect_on_association(:parent).association_foreign_key
=> "parent_id"
Child.reflect_on_association(:parent).foreign_key
=> "person_id"

When calling association_foreign_key on the association, Rails gives back parent_id, which is inappropriate for us to use in the context of detecting attribute changes on the child record. The correct key to use is person_id, ie the actual db column that ties the two models together, which is what gets passed back with foreign_key.

From the Rails docs, it seems that association_foreign_key is intended to be useful for has_and_belongs_to_many type associations (option is only defined under that section). While from the docs, it's clear foreign_key is a well-supported option across all the association types.

Looking at the history of the repo, association_foreign_key was used since the very early days of the gem. I believe it was introduced in error, and the bug hasn't been prominent enough to warrant a revisit.

✅ Ran CI on shopify core to ensure no regression

[cbothner]

I was interested in how specifically has_and_belongs_to_many associations made use of these two fields. Of course, this is irrelevant because this gem doesn't support has_and_belongs_to_many associations. Just interesting

class Store < ActiveRecord::Base
  has_and_belongs_to_many :customers
end

class Customer < ActiveRecord::Base
  has_and_belongs_to_many :stores
end

Store.reflect_on_association(:customers).foreign_key # => "store_id"
Store.reflect_on_association(:customers).association_foreign_key # => "customer_id"