Try to recommend, and catch NoSuchUserException.
If you have no data at all for a user, there's no way to make
recommendations since you know nothing about the user at all. However
in this case you typically "recommend" a selection of most popular
items to start.

2010/7/23 Matthias Böhmer <[email protected]>:
> Hello,
>
> how can I find out, if a userID is already available inside a
> DataModel? I am trying to find out if the rating data of a user is
> already known within a data model, that is updated periodically. If
> the user is not available inside the data model, I then want do treat
> him as an anonymous user. And: how can I give recommendations to a
> new, i.e. anonymous, user if I have no values for preferences. I think
> the API requires me to set preferences.
>
> Best regards,
> Matthias
>

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