Fix polymorphic preloads on NOT NULL _type columns.

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

2 files changed, 12 insertions, 10 deletions

diff --git a/activerecord/lib/active_record/associations/preloader.rb b/activerecord/lib/active_record/associations/preloader.rb
index 83637a0409..e6485b2b0c 100644
--- a/activerecord/lib/active_record/associations/preloader.rb
+++ b/activerecord/lib/active_record/associations/preloader.rb
@@ -144,7 +144,7 @@ module ActiveRecord
 
         reflection_records.each_with_object({}) do |(reflection, r_records),h|
           h[reflection] = r_records.group_by { |record|
-            association_klass(reflection, record)
+            record.association(association).klass
           }
         end
       end
@@ -163,15 +163,6 @@ module ActiveRecord
               "perhaps you misspelled it?"
       end
 
-      def association_klass(reflection, record)
-        if reflection.macro == :belongs_to && reflection.options[:polymorphic]
-          klass = record.read_attribute(reflection.foreign_type.to_s)
-          klass && klass.constantize
-        else
-          reflection.klass
-        end
-      end
-
       class AlreadyLoaded
         attr_reader :owners, :reflection
 
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