映射LINQ到SQL类域类与嵌套集合

Question

我下面的映射函数：

private static Expression<Func<ActionComment, Domain.ActionComment>> MapActionComment = 
     actionComment => new Domain.ActionComment 
     { 
     Id = actionComment.Id, 
     Comment = actionComment.Comment, 
     }; 

    private static Expression<Func<Action, Domain.Action>> MapAction = 
     action => new Domain.Domain 
     { 
     Id = action.Id, 
     Comments = action.ActionComments 
         .Select(ac => MapActionComment.Invoke(ac)) 
     }; 

    private static Expression<Func<Nc, Domain.Nc>> MapNc = 
     nc => new Domain.Nc 
       { 
       Id = nc.Id, 
       Actions = nc.Actions 
          .Select(a => MapAction.Invoke(a)), 
       }; 

而下面的LINQ到SQL查询：

_ctx.NCs 
    .Where(n => n.Id == id) 
    .Select(MapNc) 
    .SingleOrDefault(); 

产生的SQL声明是：

SELECT [t0].[Id], -- columns 
    (SELECT COUNT(*) 
    FROM [dbo].[Actions] AS [t2] 
    WHERE [t2].[NCId] = [t0].[Id] 
    ) AS [value] 
FROM [dbo].[NCs] AS [t0] 
LEFT OUTER JOIN [dbo].[Actions] AS [t1] ON [t1].[NCId] = [t0].[Id] 
WHERE [t0].[Id] = @p0 
ORDER BY [t0].[Id], [t1].[Id] 

事情是Action对象的Comments集合不会加载它的内容（直到它被缓存收集的实际使用情况）。

有什么办法可以强制Linq-to-SQL生成ONE单个语句可以获取Ncs，Actions和ActionComments吗？

Answer 1

你应该看看通过Linq2Sql的渴望加载。例如，看到这个博客： http://www.lowendahl.net/showShout.aspx?id=190

看来，有一些问题，但它应该在gerenal工作。

其实，如果有可能，我会建议转移到EntityFramework。现在，LINQ2SQL是一项传统技术。特别是，EF有预先加载了很好的支持，通过使用简单的方法Include()像这样：

_ctx.NCs 
    .Where(n => n.Id == id) 
    .Select(MapNc) 
    .Include(nc => nc.Comments) //EF to eager load relation 
    .Include(nc => nc.Actions) //EF to eager load relation 
    .SingleOrDefault();