Papertrail 6 fails to find the version primary key

[CloCkWeRX]

In some situations, doing:

contact = Contact.find(1)
contact.versions

results in invalid SQL being generated:

SELECT "versions".* FROM "versions" WHERE "versions"."item_id" = $1 AND "versions"."item_type" = $2 ORDER BY "versions"."created_at" ASC, "versions"."" ASC

The contact class has

 has_paper_trail class_name: 'Version', ignore: [:subscribed_users]

Which points to

        if ::ActiveRecord::VERSION::MAJOR >= 4
          has_many self.versions_association_name,
            lambda { order(model.timestamp_sort_order) },
            :class_name => self.version_class_name, :as => :item
        else
          has_many self.versions_association_name,
            :class_name => self.version_class_name,
            :as         => :item,
            :order      => self.paper_trail_version_class.timestamp_sort_order
        end

which does:

      def timestamp_sort_order(direction = "asc")
        [arel_table[:created_at].send(direction.downcase)].tap do |array|
          array << arel_table[primary_key].send(direction.downcase) if primary_key_is_int?
        end
      end

Here, the Version.primary_key is nil; even though it is specified in the db schema:

  create_table "versions", id: :serial, force: :cascade do |t|
    t.string "item_type", null: false
    t.integer "item_id", null: false
    t.string "event", limit: 512, null: false
    t.string "whodunnit"
    t.text "object"
    t.datetime "created_at"
    t.text "object_changes"
    t.integer "related_id"
    t.string "related_type"
    t.integer "transaction_id"
    t.index ["created_at"], name: "index_versions_on_created_at"
    t.index ["item_type", "item_id"], name: "index_versions_on_item_type_and_item_id"
    t.index ["related_id", "related_type"], name: "index_versions_on_related_id_and_related_type"
    t.index ["transaction_id"], name: "index_versions_on_transaction_id"
    t.index ["whodunnit"], name: "index_versions_on_whodunnit"
  end

Database: Postgres 9.6?

    activerecord (5.1.6.2)
      activemodel (= 5.1.6.2)
      activesupport (= 5.1.6.2)
      arel (~> 8.0)

and

    pg (0.21.0)

[CloCkWeRX]

Unique things - we do a few class_evals to extend Contact, Account classes in a funky order; but that shouldn't affect the Version table not being recognized

[CloCkWeRX]

https://github.com/rails/rails/blob/fbe2433be6e052a1acac63c7faf287c52ed3c5ba/activerecord/lib/active_record/attribute_methods/primary_key.rb#L85
is the other area where this may be an issue