we can't sort by lhs since the middle records have difference classes

and possibly different rules for finding those objects

activerecord/lib/active_record/associations/preloader/through_association.rb

@@ -46,26 +46,26 @@ def associated_records_by_owner(preloader)
             }
           end
 
-          @associated_records_by_owner = through_records.each_with_object({}) { |(lhs,middles),h|
-            preloader = middle_to_pl[middles.first]
-
-            rhs_records = middles.flat_map { |r|
-              r.send(source_reflection.name)
-            }.compact
-
-            if preloader && preloader.loaded?
-              loaded_records = preloader.preloaded_records
-              i = 0
-              record_index = loaded_records.each_with_object({}) { |r,indexes|
-                indexes[r] = i
-                i += 1
-              }
-              rs = rhs_records.sort_by { |rhs| record_index[rhs] }
-            else
-              rs = rhs_records
+          @associated_records_by_owner = through_records.each_with_object({}) { |(lhs,center),records_by_owner|
+            pl_to_middle = center.group_by { |record| middle_to_pl[record] }
+
+            records_by_owner[lhs] = pl_to_middle.flat_map do |preloader, middles|
+              rhs_records = middles.flat_map { |r|
+                r.send(source_reflection.name)
+              }.compact
+
+              if preloader && preloader.loaded?
+                loaded_records = preloader.preloaded_records
+                i = 0
+                record_index = loaded_records.each_with_object({}) { |r,indexes|
+                  indexes[r] = i
+                  i += 1
+                }
+                rhs_records.sort_by { |rhs| record_index[rhs] }
+              else
+                rhs_records
+              end
             end
-
-            h[lhs] = rs
           }
         end
 

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -47,7 +47,8 @@ def test_preload_sti_middle_relation
 
     club1 = Club.includes(:members).find_by_id club.id
     left, right = club1.members.map(&:id)
-    assert_operator left, :>, right
+    assert_equal [member1, member2].sort_by(&:id),
+                 club1.members.sort_by(&:id)
   end
 
   def make_model(name)