Extend primary key methods to support composite keys

Let's consider the cases when primary key is composite for these
methods, as this interface ends up being used in many codepaths across
Rails.

Author: paarthmadan

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -38,17 +38,29 @@ def id=(value)
 
       # Queries the primary key column's value.
       def id?
-        query_attribute(@primary_key)
+        if self.class.composite_primary_key?
+          @primary_key.all? { |col| query_attribute(col) }
+        else
+          query_attribute(@primary_key)
+        end
       end
 
       # Returns the primary key column's value before type cast.
       def id_before_type_cast
-        attribute_before_type_cast(@primary_key)
+        if self.class.composite_primary_key?
+          @primary_key.map { |col| attribute_before_type_cast(col) }
+        else
+          attribute_before_type_cast(@primary_key)
+        end
       end
 
       # Returns the primary key column's previous value.
       def id_was
-        attribute_was(@primary_key)
+        if self.class.composite_primary_key?
+          @primary_key.map { |col| attribute_was(col) }
+        else
+          attribute_was(@primary_key)
+        end
       end
 
       # Returns the primary key column's value from the database.
@@ -61,7 +73,11 @@ def id_in_database
       end
 
       def id_for_database # :nodoc:
-        @attributes[@primary_key].value_for_database
+        if self.class.composite_primary_key?
+          @primary_key.map { |col| @attributes[col].value_for_database }
+        else
+          @attributes[@primary_key].value_for_database
+        end
       end
 
       private

activerecord/test/cases/primary_keys_test.rb

@@ -53,6 +53,24 @@ def test_to_key_with_primary_key_after_destroy
     assert_equal [1], topic.to_key
   end
 
+  def test_id_was
+    topic = Topic.find(1)
+    assert_equal 1, topic.id
+
+    topic.id = 3
+
+    assert_equal 1, topic.id_was
+    assert_equal 3, topic.id
+  end
+
+  def test_id?
+    topic = Topic.find(1)
+
+    assert_changes("topic.id?", from: true, to: false) do
+      topic.id = nil
+    end
+  end
+
   def test_integer_key
     topic = Topic.find(1)
     assert_equal(topics(:first).author_name, topic.author_name)
@@ -413,6 +431,38 @@ def test_assigning_a_non_array_value_to_model_with_composite_primary_key_raises
     end
   end
 
+  def test_id_was_composite
+    book = cpk_books(:cpk_great_author_first_book)
+    book_id = book.id
+
+    assert_not_equal [42, 42], book_id
+
+    book.id = [42, 42]
+
+    assert_equal book_id, book.id_was
+    assert_equal [42, 42], book.id
+  end
+
+  def test_id_predicate_composite
+    book = cpk_books(:cpk_great_author_first_book)
+
+    valid_id = [42, 42]
+
+    invalid_ids = [
+      [42, nil],
+      [nil, 42],
+      [nil, nil],
+    ]
+
+    invalid_ids.each do |invalid_id|
+      book.id = valid_id
+
+      assert_changes("book.id?", from: true, to: false) do
+        book.id = invalid_id
+      end
+    end
+  end
+
   def test_primary_key_issues_warning
     model = Class.new(ActiveRecord::Base) do
       def self.table_name