added failing spec to show how both symbol and string keys are causing problems

test/unit/active_model/serializer/has_one_and_has_many_test.rb

@@ -0,0 +1,27 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class HasOneAndHasManyTest < Minitest::Test
+      def setup
+        @post = SpecialPost.new({ title: 'T1', body: 'B1'})
+        @post_serializer = SpecialPostSerializer.new(@post)
+      end
+
+      def teardown
+      end
+
+      def test_side_load_has_one_and_has_many_in_same_array
+        assert_equal({
+          "post" => {
+            title: 'T1',
+            body: 'B1',
+            'comment_ids' => @post.comments.map { |c| c.object_id },
+            'special_comment_id' => @post_serializer.special_comment.object_id,
+          },
+          "comments" => [{ content: 'C1' }, { content: 'C2' }, { content: 'special' }]
+        }, @post_serializer.as_json(root: 'post'))
+      end
+    end
+  end
+end