Add a test case that eager-loading with a polymorphic association and using `exists?`

This test covers the case of 02da8aea.

Previously `exists?` was always eager-loading the includes values. But
now it is eager-loaded only when necessary since 07a611e0.

So the case of the eager-loading had not covered in the test.

1 files changed, 4 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 559f0e9338..9afe6a893c 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1501,6 +1501,10 @@ class EagerAssociationTest < ActiveRecord::TestCase
     assert_equal posts(:welcome), post
   end
 
+  test "eager-loading with a polymorphic association and using the existential predicate" do
+    assert_equal true, authors(:david).essays.eager_load(:writer).exists?
+  end
+
   # CollectionProxy#reader is expensive, so the preloader avoids calling it.
   test "preloading has_many_through association avoids calling association.reader" do
     ActiveRecord::Associations::HasManyAssociation.any_instance.expects(:reader).never