Minor - P4 In stitch, we rely on invoking UpdateDriver::update() to return a FieldRefSet indicating which fields were modified by the update operation. To make this possible, we're using a patched version of 3.5.8 here: https://github.com/mpobrien/mongo/commits/stitch-update-doc
In newer versions of the code, the modified fields are no longer available from update(). However, we'd ideally like to be to compiling against a more current version of mongo so that we get all the bug fixes and new update operators. If there was a way the modified fields could be exposed in some form in the current update code, it might make that possible for us.