I've been looking for a solution regarding my question in the subject. There has to be a better way than doing an api call to check for existing users, do a put to update every single field (even though I only want to change just 1) or a push to create a new user.
Is there a way to send a contact id and add it to a list simply?
Our API use to clear out all of the data that was not submitted in fact our documentation states that is what it does. I just tested this and it appears that has changed. I will dig back through our updates to see when this changed.
Given that change it is still a good practice to GET the contact before you do a PUT and update the listIDs because there are some circumstances that can cause the contactID to change. It is considered a bad practice to store the contactID and use that to submit changes without first GETting the current data.