How do I set a column via Connect Rest API which is a foreign key to another table?

Environment:

May 2015 and newer

Resolution:

To set the value of a column that is a foreign key to another table, the id attribute must be set for the column in the POST request.

For example, if a custom object contains a column called contact_id and this column is a foreign key to the Contacts table, a POST request to set this column would look like the following:

{

   "contact_id": {

      "id": 292

   }

}