public JsonResult MoveDates(Guid sourceDozId, Guid targetDozId)
        {
            var sourceMember = Db.Members.SingleOrDefault(x => x.Id == sourceDozId);
            var targetMember = Db.Members.SingleOrDefault(x => x.Id == targetDozId);

            // alle termine, des abgebenden Dozenten
            var dates = Db.ActivityDates.Where(x => x.Hosts.Any(h => h.Id == sourceMember.Id) && x.End > DateTime.Now).OrderBy(x => x.Begin).ToList();

            // Umhängen
            foreach (var date in dates)
            {
                date.Hosts.Remove(sourceMember);
                // nur hinzufügen, falls nicht eh schon dabe
                if (!date.Hosts.Contains(targetMember))
                {
                    date.Hosts.Add(targetMember);
                }
            }
            Db.SaveChanges();

            // jetzt noch eine Mail senden
            var userService = new UserInfoService();

            var mailModel = new MemberMoveDateMailModel
            {
                SourceUser   = userService.GetUser(sourceMember.UserId),
                TargetUser   = userService.GetUser(targetMember.UserId),
                SourceMember = sourceMember,
                TargetMember = targetMember,
                AdminMember  = GetMyMembership(),
                User         = GetCurrentUser(),
                Dates        = dates
            };

            var mail = new MailController();

            mail.MemberMoveDateEMail(mailModel).Deliver();


            // Redirect
            return(Json(new { result = "Redirect", url = Url.Action("Index") }));
        }