MongoDB提示Only fields are allowed in a $sort

var query = colleciton.AsQueryable().OrderByDescending(a => (a.Telephone == null) ? a.MobilePhone: a.Telephone);

var query = colleciton.AsQueryable()
                .Select(a => new { Customer = a, SortBy = a.Telephone == null ? a.MobilePhone : a.Telephone })
                .OrderByDescending(a => a.SortBy)
                .Select(a => a.Customer);