Question

我有一个SQL数据库建模这个类。

Public Class Language
  Public Property Name As String
  Public Property Articles As List(Of Article)
End Class

Public Class Article
  Public Property Dated as DateTime?
End Class

我想获得一份语言列表以及上一篇文章的日期

但如果文章有效日期，我必须检查文章是否有项目。

查询的正确方法是什么？到目前为止我已经这样做了：

  Dim query = From x In context.Languages
              Order By x.Name
              Select New MyViewModel() With {
                .Name = x.Name,
                .LastArticleDate = If(x.Articles.Count <> 0 AndAlso
                   x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault IsNot Nothing AndAlso
                   x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault.Dated.HasValue,
                   x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault.Dated.Value.ToShortDateString,
                   String.Empty)
              }

Answer 1

我假设Article类看起来像这样：

Public Class Article
  Public Property Dated as DateTime?
End Class

您应该使用EF从数据库获取DateTime?，然后执行ToShortDateString作为LINQ to Objects查询。 EF无法将ToShortDateString转换为正确的SQL。

Dim query = From x In context.Languages
            Order By x.Name
            Select New With {
              .Name = x.Name,
              .LastArticleDate = x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault()
            }

Dim results = From x In query.AsEnumerable()
              Select New MyViewModel() With {
                .Name = x.Name,
                .LastArticleDate = If(!x.LastArticleDate.HasValue, "", x.LastArticleDate.ToShortDateString())
              }

检查LINQ中可能为null的值

1 个答案: