Fix polymorphic preloads on NOT NULL _type columns.

Defer to Association#klass instead of having a custom/duplicate code.

1 files changed, 11 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index e59161fc5b..d594697280 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -235,6 +235,17 @@ class EagerAssociationTest < ActiveRecord::TestCase
     end
   end
 
+  def test_finding_with_includes_on_empty_polymorphic_type_column
+    sponsor = sponsors(:moustache_club_sponsor_for_groucho)
+    sponsor.update!(sponsorable_type: '', sponsorable_id: nil) # sponsorable_type column might be declared NOT NULL
+    sponsor = assert_queries(1) do
+      assert_nothing_raised { Sponsor.all.merge!(:includes => :sponsorable).find(sponsor.id) }
+    end
+    assert_no_queries do
+      assert_equal nil, sponsor.sponsorable
+    end
+  end
+
   def test_loading_from_an_association
     posts = authors(:david).posts.merge(:includes => :comments, :order => "posts.id").to_a
     assert_equal 2, posts.first.comments.size