Define ActiveRecord::Base#id API for composite primary key models

Given a model with a composite primary key:
```ruby
class Order < ActiveRecord::Base
  self.primary_key = [:shop_id, :id]
end
```

`ActiveRecord::Base#id` method will return an array of values for every
column of the primary key.

```ruby
order = Order.create!(shop_id: 1, id: 2)
order.id # => [1, 2]
```

The `id` column is accessible through the `read_attribute` method:

```ruby
order.read_attribute(:id) # => 2
```

Author: nvasilevski

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -16,7 +16,9 @@ def to_key
 
       # Returns the primary key column's value.
       def id
-        _read_attribute(@primary_key)
+        return _read_attribute(@primary_key) unless @primary_key.is_a?(Array)
+
+        @primary_key.map { |pk| _read_attribute(pk) }
       end
 
       # Sets the primary key column's value.
@@ -120,12 +122,20 @@ def get_primary_key(base_name) # :nodoc:
           #
           #   Project.primary_key # => "foo_id"
           def primary_key=(value)
-            @primary_key        = value && -value.to_s
+            @primary_key        = derive_primary_key(value)
             @quoted_primary_key = nil
             @attributes_builder = nil
           end
 
           private
+            def derive_primary_key(value)
+              return unless value
+
+              return -value.to_s unless value.is_a?(Array)
+
+              value.map { |v| -v.to_s }.freeze
+            end
+
             def inherited(base)
               super
               base.class_eval do

activerecord/lib/active_record/attribute_methods/read.rb

@@ -29,7 +29,7 @@ def read_attribute(attr_name, &block)
         name = attr_name.to_s
         name = self.class.attribute_aliases[name] || name
 
-        name = @primary_key if name == "id" && @primary_key
+        name = @primary_key if name == "id" && @primary_key && !@primary_key.is_a?(Array)
         @attributes.fetch_value(name, &block)
       end
 

activerecord/lib/active_record/fixtures.rb

@@ -769,7 +769,8 @@ def [](key)
     def find
       raise FixtureClassNotFound, "No class attached to find." unless model_class
       object = model_class.unscoped do
-        model_class.find(fixture[model_class.primary_key])
+        pk_clauses = fixture.slice(*Array(model_class.primary_key))
+        model_class.find_by!(pk_clauses)
       end
       # Fixtures can't be eagerly loaded
       object.instance_variable_set(:@strict_loading, false)

activerecord/test/cases/primary_keys_test.rb

@@ -10,6 +10,7 @@
 require "models/mixed_case_monkey"
 require "models/dashboard"
 require "models/non_primary_key"
+require "models/cpk"
 
 class PrimaryKeysTest < ActiveRecord::TestCase
   fixtures :topics, :subscribers, :movies, :mixed_case_monkeys
@@ -331,6 +332,8 @@ class CompositePrimaryKeyTest < ActiveRecord::TestCase
 
   self.use_transactional_tests = false
 
+  fixtures :cpk_books, :cpk_orders
+
   def setup
     @connection = ActiveRecord::Base.connection
     @connection.schema_cache.clear!
@@ -387,6 +390,19 @@ def test_dumping_composite_primary_key_out_of_order
     schema = dump_table_schema "barcodes_reverse"
     assert_match %r{create_table "barcodes_reverse", primary_key: \["code", "region"\]}, schema
   end
+
+  def test_model_with_a_composite_primary_key
+    assert_equal(["author_id", "number"], Cpk::Book.primary_key)
+    assert_equal(["shop_id", "id"], Cpk::Order.primary_key)
+  end
+
+  def test_id_is_not_defined_on_a_model_with_composite_primary_key
+    book = cpk_books(:cpk_great_author_first_book)
+    order = cpk_orders(:cpk_groceries_order_1)
+
+    assert_equal([book.author_id, book.number], book.id)
+    assert_equal([order.shop_id, order.read_attribute(:id)], order.id)
+  end
 end
 
 class PrimaryKeyIntegerNilDefaultTest < ActiveRecord::TestCase

activerecord/test/fixtures/cpk_books.yml

@@ -0,0 +1,20 @@
+_fixture:
+  model_class: Cpk::Book
+
+cpk_great_author_first_book:
+  author_id: 1
+  number: 1
+  title: "The first book"
+  revision: 1
+
+cpk_great_author_second_book:
+  author_id: 1
+  number: 2
+  title: "The second book"
+  revision: 1
+
+cpk_famous_author_first_book:
+  author_id: 2
+  number: 1
+  title: "Ruby on Rails"
+  revision: 1

activerecord/test/fixtures/cpk_orders.yml

@@ -0,0 +1,17 @@
+_fixture:
+  model_class: Cpk::Order
+
+cpk_groceries_order_1:
+  id: 1
+  shop_id: 1
+  status: "paid"
+
+cpk_groceries_order_2:
+  id: 2
+  shop_id: 1
+  status: "paid"
+
+cpk_coffee_order_1:
+  id: 3
+  shop_id: 2
+  status: "cancelled"

activerecord/test/models/cpk.rb

@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+require_relative "cpk/book"
+require_relative "cpk/order"

activerecord/test/models/cpk/book.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+module Cpk
+  class Book < ActiveRecord::Base
+    self.table_name = :cpk_books
+    self.primary_key = [:author_id, :number]
+  end
+end

activerecord/test/models/cpk/order.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+module Cpk
+  class Order < ActiveRecord::Base
+    self.table_name = :cpk_orders
+    self.primary_key = [:shop_id, :id]
+  end
+end

activerecord/test/schema/schema.rb

@@ -239,6 +239,19 @@
     t.references :citation
   end
 
+  create_table :cpk_books, primary_key: [:author_id, :number], force: true do |t|
+    t.integer :author_id
+    t.integer :number
+    t.string :title
+    t.integer :revision
+  end
+
+  create_table :cpk_orders, primary_key: [:shop_id, :id], force: true do |t|
+    t.integer :shop_id
+    t.integer :id
+    t.string :status
+  end
+
   create_table :clothing_items, force: true do |t|
     t.string :clothing_type
     t.string :color