diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index 0fb21dce85..10d6bbfdf5 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -392,6 +392,17 @@ def test_distinct_count_with_group_by_and_order_and_limit
     assert_equal({ 6 => 2 }, Account.group(:firm_id).distinct.order("1 DESC").limit(1).count)
   end
 
+  def test_count_for_a_composite_primary_key_model
+    book = cpk_books(:cpk_great_author_first_book)
+    assert_equal(1, Cpk::Book.where(author_id: book.author_id, number: book.number).count)
+  end
+
+  def test_group_by_count_for_a_composite_primary_key_model
+    book = cpk_books(:cpk_great_author_first_book)
+    expected = { book.author_id => Cpk::Book.where(author_id: book.author_id).count }
+    assert_equal(expected, Cpk::Book.where(author_id: book.author_id).group(:author_id).count)
+  end
+
   def test_should_group_by_summed_field_having_condition
     c = Account.group(:firm_id).having("sum(credit_limit) > 50").sum(:credit_limit)
     assert_nil        c[1]