Fix replacing foreign key for a CPK association by id attribute

[nvasilevski]

Given a model associated with a composite primary key by id attribute, for example:

Order.primary_key = [:shop_id, :id]
OrderAgreement.primary_key = :id

OrderAgreement.belongs_to :order, primary_key: :id

Assigning order to an OrderAgreement object will replace the order_id foreign key:

order = Order.last # => #<Order id: 1, shop_id: 2>

order_agreement = OrderAgreement.new(order: order).save
order_agreement.order_id # => 1

Implementation details

Same as in #47923 we can't use public_send which results in the id method being called which for a composite primary key model returns a composite primary key values instead of the id column value. We are using _read_attribute instead

[nvasilevski]

I'm going to have a look on these failures

[nvasilevski]

I just wanted to add a bit of context on this change. This is something we have already discussed in
https://github.com/rails/rails/pull/47378/files#r1104639481
and
#47230 (comment)

Hardcoded :id value was incorrect but was working because :id through object.public_send(:id) translates to whichever column being used as a primary key. So if the primary key is title column - object.public_send(:id) returns the title value. But it wasn't working well with a composite primary key where id column is a part of the key so in order for it to work properly we changed it to be dynamically derived so if the record's primary key is title the compute_primary_key method returns "title"