Question

我们公司正在转向实体框架，它与我们的旧系统有很大不同，所以我们有点迷失。

在这种情况下，我们需要一个左连接。我可以确认我应该返回1行我需要的结果，但我似乎无法编写连接查询以成功拉出任何行。它只显示返回0行。

我们尝试了3种不同的方法，每次都失败了。我觉得我们很接近，但似乎无法让它发挥作用。

1）此方法不会抛出异常，但会返回0结果。

    var joinedHistoricalData = from loc in locationQuery
                               join hd in historicalData on loc.LocationFieldLocId equals hd.LocationId
                              select new HistoricalDataViewModel
                               {
                                   Id = hd.Id,
                                   LocId = (loc.LocationFieldLocId == null ? 0: (int)loc.LocationFieldLocId) ,
                                   DivisionId = (loc.LocationFieldDivisionId == null ? 0 : (int)loc.LocationFieldDivisionId),
                                   LocationPartName = loc.LocationFieldName,
                                   LocationPartAddress1 = loc.LocationFieldAddressLine1,
                                   LocationPartAddress2 = loc.LocationFieldAddressLine2,
                                   LocationPartDescription = loc.LocationFieldDescription,
                                   Entries = hd.Entries,
                                   ModifiedDate =  hd.ModifiedDate ,
                                   ModifiedBy = String.Empty
                                };

2）按照指南我们稍微更改了一下查询以添加DefaultIfEmpty（）方法。也返回0结果。

var joinedHistoricalData = from loc in locationQuery
                                   join hd in historicalData on loc.LocationFieldLocId equals hd.LocationId  into hdTemp
                                   from hdSub in hdTemp.DefaultIfEmpty()
                                  select new HistoricalDataViewModel
                                   {
                                       Id = hdSub .Id,
                                       LocId = (loc.LocationFieldLocId == null ? 0: (int)loc.LocationFieldLocId) ,
                                       DivisionId = (loc.LocationFieldDivisionId == null ? 0 : (int)loc.LocationFieldDivisionId),
                                       LocationPartName = loc.LocationFieldName,
                                       LocationPartAddress1 = loc.LocationFieldAddressLine1,
                                       LocationPartAddress2 = loc.LocationFieldAddressLine2,
                                       LocationPartDescription = loc.LocationFieldDescription,
                                       Entries = hdSub .Entries,
                                       ModifiedDate =  hdSub .ModifiedDate ,
                                       ModifiedBy = String.Empty
                                    };

3）另一个指南提到我们应该指定左连接空对象，所以在DefaultIfEmpty（）方法中我们将它添加为一个参数，它完全崩溃说LINQ无法创建实体框架复杂对象。

new ef_HistoricalData{Id = 0,
                      Entries= 0,
                      DivisionId = (loc.LocationFieldDivisionId == null ? 0 : (int)loc.LocationFieldDivisionId),
                      LocId = (loc.LocationFieldLocId == null ? 0 : (int)loc.LocationFieldLocId),
                      ModifiedById = null,
                      ModifiedDate = DateTime.UtcNow 
                     }

我不知道该怎么做。我在做一个Leftjoin错了吗？

*注意，这里需要完整或左连接，右边或内部将返回0结果我相信）

实体框架左连接不提取数据。

0 个答案: