Fix associations associated with a CPK model by id attribute

Given a model associated with a composite primary key by `id` attribute,
for example:
```ruby
Order.primary_key = [:shop_id, :id]
OrderAgreement.primary_key = :id

Order.has_many :order_agreements, primary_key: :id
```

Accessing the association should perform queries using
the `id` attribute value and not the `id` as Order's composite primary key.

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

order.order_agreements.to_a
```

activerecord/lib/active_record/associations/association_scope.rb

@@ -62,7 +62,7 @@ module ActiveRecord
           table = reflection.aliased_table
           primary_key_foreign_key_pairs = primary_key.zip(foreign_key)
           primary_key_foreign_key_pairs.each do |join_key, foreign_key|
-            value = transform_value(owner[foreign_key])
+            value = transform_value(owner._read_attribute(foreign_key))
             scope = apply_scope(scope, table, join_key, value)
           end
 

activerecord/lib/active_record/reflection.rb

@@ -550,7 +550,7 @@ module ActiveRecord
       end
 
       def join_id_for(owner) # :nodoc:
-        Array(join_foreign_key).map { |key| owner[key] }
+        Array(join_foreign_key).map { |key| owner._read_attribute(key) }
       end
 
       def through_reflection

activerecord/test/cases/associations_test.rb

@@ -35,6 +35,7 @@ require "models/essay"
 require "models/member"
 require "models/membership"
 require "models/sharded"
+require "models/cpk"
 require "models/member_detail"
 require "models/organization"
 
@@ -42,7 +43,8 @@ require "models/organization"
 class AssociationsTest < ActiveRecord::TestCase
   fixtures :accounts, :companies, :developers, :projects, :developers_projects,
            :computers, :people, :readers, :authors, :author_addresses, :author_favorites,
-           :comments, :posts, :sharded_blogs, :sharded_blog_posts, :sharded_comments, :sharded_tags, :sharded_blog_posts_tags
+           :comments, :posts, :sharded_blogs, :sharded_blog_posts, :sharded_comments, :sharded_tags, :sharded_blog_posts_tags,
+           :cpk_orders
 
   def test_eager_loading_should_not_change_count_of_children
     liquid = Liquid.create(name: "salty")
@@ -136,6 +138,14 @@ class AssociationsTest < ActiveRecord::TestCase
     assert_equal(blog_post, comment.blog_post)
   end
 
+  def test_belongs_to_a_cpk_model_by_id_attribute
+    order = cpk_orders(:cpk_groceries_order_1)
+    _order_shop_id, order_id = order.id
+    agreement = Cpk::OrderAgreement.create(order_id: order_id, signature: "signed")
+
+    assert_equal(order, agreement.order)
+  end
+
   def test_belongs_to_a_model_with_composite_primary_key_uses_composite_pk_in_sql
     comment = sharded_comments(:great_comment_blog_post_one)
 
@@ -164,6 +174,14 @@ class AssociationsTest < ActiveRecord::TestCase
     assert_includes(comments, sharded_comments(:great_comment_blog_post_one))
   end
 
+  def test_cpk_model_has_many_records_by_id_attribute
+    order = cpk_orders(:cpk_groceries_order_1)
+    _order_shop_id, order_id = order.id
+    agreements = 2.times.map { Cpk::OrderAgreement.create(order_id: order_id, signature: "signed") }
+
+    assert_equal(agreements.sort, order.order_agreements.to_a.sort)
+  end
+
   def test_has_many_association_from_a_model_with_query_constraints_different_from_the_association
     blog_post = sharded_blog_posts(:great_post_blog_one)
     blog_post = Sharded::BlogPostWithRevision.find(blog_post.id)

activerecord/test/models/cpk.rb

@@ -2,3 +2,4 @@
 
 require_relative "cpk/book"
 require_relative "cpk/order"
+require_relative "cpk/order_agreement"

activerecord/test/models/cpk/order.rb

@@ -4,5 +4,7 @@ module Cpk
   class Order < ActiveRecord::Base
     self.table_name = :cpk_orders
     self.primary_key = [:shop_id, :id]
+
+    has_many :order_agreements, primary_key: :id
   end
 end

activerecord/test/models/cpk/order_agreement.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+module Cpk
+  # This is a non composite primary key model that is associated with `Cpk::Order` via `id` only.
+  class OrderAgreement < ActiveRecord::Base
+    self.table_name = :cpk_order_agreements
+
+    belongs_to :order, primary_key: :id # foreign key is derived as `order_id`
+  end
+end

activerecord/test/schema/schema.rb

@@ -252,6 +252,13 @@ ActiveRecord::Schema.define do
     t.string :status
   end
 
+  create_table :cpk_order_agreements, force: true do |t|
+    t.integer :order_id
+    t.string :signature
+
+    t.index :order_id
+  end
+
   create_table :paragraphs, force: true do |t|
     t.references :book
   end