LINQ，無法加入字元串

我有一個使用者列表，每個使用者都有問題列表。在我的模型中，問題列表應該通過逗號以字元串形式出現。我嘗試：

public List<ITW2012Mobile.ViewModels.AdminSurveyReportModel> SurveyReportList()
{
   var q = from i in _dbContext.Users
           where i.UserId != null
           select new ITW2012Mobile.ViewModels.AdminSurveyReportModel()
           {
               FirstName = i.FirstName,
               LastName = i.LastName,
               Question4 = String.Join(", " , (from a in _dbContext.MultipleQuestions where a.MultipleQuestionType.KEY == MultipleQuestionKeys.BENEFITS select a.Question).ToArray())
           };
   return q.ToList();
}

public class AdminSurveyReportModel
{
   public string FirstName { get; set; }
   public string LastName { get; set; }
   public string Question4 { get; set; }
}

當然，我得到錯誤：

LINQ to Entities 無法辨識方法 ‘System.String Join(System.String, System.String

$$ $$)’ 方法，並且該方法不能翻譯成商店表達式。

如何正確獲取？

我建議在string.Join本地進行操作，而不是使用AsEnumerable：

var q = from i in _dbContext.Users
       where i.UserId != null
       select new
       {
           FirstName = i.FirstName,
           LastName = i.LastName,
           Question4Parts = _dbContext.MultipleQuestions
                                      .Where(a => a.MultipleQuestionType.KEY == 
                                                  MultipleQuestionKeys.BENEFITS)
                                      .Select(a => a.Question)
       };

return q.AsEnumerable()
       .Select(x => new ITW2012Mobile.ViewModels.AdminSurveyReportModel
                    {
                        FirstName = x.FirstName,
                        LastName = x.LastName,
                        Question4 = string.Join(", ", x.Question4Parts)
                    })
       .ToList();

引用自：https://stackoverflow.com/questions/9647039