Managed metadata document column not always updated

Question

Im trying to update a column for a document in a document library in Sharepoint Online with the Sharepoint Rest API. I'm not very familiar with Sharepoint in general, but after a few days of struggle I've finally managed to update some managed metadata.

I've successfully been able to update a few documents with the following, but I'm getting inconsistent results.

payload:

{
  "MyCoolField": {
    "Label": "20.0015-LANGMOEN",
    "TermGuid": "6ed9cf13-5ffe-466b-873c-3485f9a651cb",
    "WssId": 25,
    "__metadata": {
      "type": "SP.Taxonomy.TaxonomyFieldValue"
    }
  },
  "__metadata": {
    "type": "SP.Data.Shared_x0020_DocumentsItem"
  }
}

headers:

headers = {
    "Content-Length": str(len(json.dumps(payload))),
    "Accept": "application/json; odata=verbose",
    "Content-Type": "application/json; odata=verbose",
    "X-RequestDigest": jsonDigestValue,
    "IF-MATCH": "*",
    "X-HTTP-Method": "MERGE"
}

I'm posting this to https://mysite.sharepoint.com/teams/myTeam/_api/Web/GetFileByServerRelativeUrl('/teams/myTeam/Shared Documents/dir/afile.pdf')/ListItemAllFields and get a 204 response. In some cases the document metadata gets updated with the term label and in some cases it doesn't. I've also tried to update one that already has a value. When I do that, nothing happens, but I still get a 204 response.

I don't understand what's happening. Is there some hidden mechanism I'm not aware of? Any help would be greatly appreciated.

Answer 1

I am surprised if you get it to work at all, the ListItemAllFields method returns a Read-Only copy of the ListItem (see the R/W table here). Use the values you get from here to get the Item ID and save that in your code, then use that to do your MERGE to https://mysite.sharepoint.com/teams/myTeam/_api/web/lists/getbytitle('Shared Documents')/items(<item id>) (list item ref here).