public static User find_user_of_contribution(Contribution c)
{
naturenet_dataclassDataContext db = new naturenet_dataclassDataContext();
var users = from mappings in db.Collection_Contribution_Mappings
where mappings.contribution_id == c.id
select mappings.Collection.User;
if (users == null) return null;
if (users.Count() == 0) return null;
return users.First<User>();
}