Error updating document metadata with REST Flow

Question

So I am beating my head against the wall and have done more searching on SE than I ever cared to. I am trying to update the Modified By field for a document in a Document Library on a Sharepoint team site from a flow triggered by a form submission. Basically I want the user that submitted the form to show as the last modified by user when using highlighted content web parts. I figured it should be pretty simple, but here is my flow card with current settings:

When I run this in this way, I get the following error:

"An entry without a type name was found, but no expected type was specified. To allow entries without type information, the expected type must also be specified when the model is specified."

Interestingly enough, when I use the POST method instead of the PATCH method, I get this error:

"The parameter _metadata does not exist in method GetById"

Strange since I am using method GetByTitle, huh? Now I know I have hard coded the item number and the Editor ID, but that is just for testing purposes. I plan on parameterizing them before I go live with this flow (which was supposed to be today!)

Any idea why I am having so many issues? I'll even take a different way to update this field if you all have one. Thanks in advance folks!

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UPDATE (05/05/2020):

So I came pack to this project and was able to get the flow to complete through this step, but it doesn't actually update the EditorId field. Here is the results card that says should be working:

You'll notice this only works using the PATCH method. When I try the POST method, I still get this error:

"The parameter __metadata does not exist in method GetById."

For your information, here is the results card that failed in the flow:

So, basically, I am still stumped and would love any assistance you find folks are able to provide. Thanks!

Answer 1

As far I know you cannot use this REST endpoint for modifying Modified by, because these fields are special and are automatically filled by SharePoint itself. But you can use diferent endopoint ValidateUpdateListItem - something like _api/web/lists(guid'libraryGUID')/items(itemID)/ValidateUpdateListItem. This endpoint is a little bit trickier to use, but it allows you to modify "system" fields.
More about using this endpoint:

• Update 'Created By' and 'Modified By' fields.
• Lists and list items REST API reference

Please do not forget mark this as answer if it helps you.