Django user interacition with other users?

In a Django web app, let's say there is a prediction cast by the user Daniel. In the prediction, there is an accept button, any User can accept this prediction.

What I want to know is that if User Andrew accepts the offer, how can I store that in Daniel's prediction class? (If there is an accepted_by field in Daniel's prediction class, how can I change that field to Andrew, the actual User object not just a string, when User Andrew accepts that prediction)?

Thanks