Support scoped ActiveRecord associations when joining for sorting

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• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

Features:

It would be good to support scoped ActiveRecord associations when resources are queried for sorting.

Example

class Contact < ApplicationRecord
  has_many :addresses
  has_one :primary_address, -> { where(primary: true) }, class_name: 'Address'
end

class Address < ApplicationRecord
  belongs_to :contact
end

class ContactResource < JSONAPI::Resource
  has_one :primary_address, class_name: 'Address'

  class << self
      def sortable_fields(context)
        super + %i[primary_address.street]
      end
    end
end

If contact is queried with sort=primary_address.street, the resulting query on the current master branch would ignore the scope of the association.

The SQL query would look something like this:

SELECT  "contacts".* FROM contacts LEFT JOIN addresses AS primary_address_sorting ON primary_address_sorting.contact_id = contacts.id ORDER BY primary_address_sorting.street asc 

My proposal would be to use the native syntax of ActiveRecord's joins instead of the current stitching of queries. Thus, _build_joins(https://github.com/cerebris/jsonapi-resources/blob/5dfeb0f741e3fc98ee9786f7eb182b15f391ca63/lib/jsonapi/active_relation_resource_finder.rb#L431-L446) would return something like [:primary_address]. For longer association chains, it would return something like [:book => [:author => [:address]]].

You can find a WIP PR for this here: https://github.com/HQTrust/jsonapi-resources/pull/1

I'd be very happy to get your feedback on this to eventually create an upstream PR. Because of the approach to structure the joins, I don't think it is possible to alias them as #{association.name}_sorting anymore, which would make this a breaking change for some users. If you have an example for this, maybe you can provide me with one so I can better understand the impact.

Start in lib/jsonapi/active_relation_resource_finder.rb, especially _build_joins at the referenced lines, and compare the proposed structure with the linked WIP PR. Verify how scoped ActiveRecord associations and longer association chains should be represented, then assess the stated aliasing and breaking-change concern. Done means sorting through scoped associations preserves their scope with the intended join structure.