Is it possible to set a Person or Group field through the REST API when the user is not listed on /sites/mysite/_api/web/siteusers?

Question

My goal is to set a Person or Group field value on a new record to a user who may not have logged into the subsite "mysite" (and does not exist at endpoint #2 below), but who does exist at root endpoint, #1 below.

For example, John Smith exists with Id 23 at /sites/_api/web/siteusers, but he does not exist at all at /sites/mysite/_api/web/siteusers. I want to store John Smith in a Person or Group field in a list located in /mysite. John Smith has no Id for that site, however, hence the question.

I'm working with the SharePoint Online REST API using endpoints similar to these:

1 - tenant.sharepoint.com/sites/_api/web/siteusers

2 - tenant.sharepoint.com/sites/mysite/_api/web/siteusers

3 - tenant.sharepoint.com/sites/mysite/_api/web/lists/GetByTitle('MyList')/items

I don't think it is relevant, but the context is a custom web part built with the SharePoint Framework. The web part will be hosted on a modern page within /sites/mysite and is being tested on the tenant workbench (not local) with support files being served from my local machine.

Answer 1

If you want a reference to a SP.User stored on a SP.ListItem the user must exist in the SP.UserCollection siteusers of the specific SP.Web (i.e. tenant.sharepoint.com/sites/web/subweb/_api/web/siteusers).

The reason for this is that only the local reference to the Id in the siteusers-list is stored when a user is referenced (using a SP.FieldLookup).

So you'll need to ensure the user exists on the corresponging SP.Web. This is done using the InsureUser-Method of the SP.Web. I.e. by sending the correct POST request to http://<site url>/_api/web/ensureuser