linq - 亚音速 3.0 和 linq

Question

我正在玩弄 Subsonic 3.0 SimpleRepository 并尝试使用一个 linq 查询获取菜单和菜单项，但菜单项始终为空

菜单

public class Menu
{
    public Menu()
    {
        MenuId = 0;
        MenuName = "";
        MenuItems = null;
    }
    public int MenuId { get; set; }
    public string MenuName { get; set; }
    public MenuItem MenuItems { get; set; }
}

菜单项

public class MenuItem
{
    public MenuItem()
    {
        MenuItemId = 0;
        MenuId = 0;
        MenuItemName = "";
    }
    public int MenuItemId { get; set; }
    public int MenuId { get; set; }
    public string MenuItemName { get; set; }
}

Linq 查询

var menus =  from m in _repo.All<Menu>()
             from mi in _repo.All<MenuItem>()
             where m.MenuItems.MenuItemId == mi.MenuItemId
             select new Menu
             {
                 MenuId = m.MenuId,
                 MenuName = m.MenuName,
                 MenuItems = {
                             MenuItemId = mi.MenuItemId,
                             MenuItemName = mi.MenuItemName
                        }
             };

有人能告诉我我做错了什么吗？

Answer 1

我想我已经找到了这个问题的实际答案。我一直在 SubSonic 源代码中翻找，发现在将数据读取器映射到对象时使用了两种类型的对象投影:一种用于匿名类型和分组，另一种用于其他所有类型:

这是一个片段:SubSonic.Linq.Structure.DbQueryProvider 的第 269 - 298 行

IEnumerable<T> result;
Type type = typeof (T);
//this is so hacky - the issue is that the Projector below uses Expression.Convert, which is a bottleneck
//it's about 10x slower than our ToEnumerable. Our ToEnumerable, however, stumbles on Anon types and groupings
//since it doesn't know how to instantiate them (I tried - not smart enough). So we do some trickery here.
    if (type.Name.Contains("AnonymousType") || type.Name.StartsWith("Grouping`") || type.FullName.StartsWith("System.")) {
    var reader = _provider.ExecuteReader(cmd);
    result = Project(reader, query.Projector);
    } else
    {
        using (var reader = _provider.ExecuteReader(cmd))
        {
            //use our reader stuff
            //thanks to Pascal LaCroix for the help here...
            var resultType = typeof (T);
            if (resultType.IsValueType)
            {
                result = reader.ToEnumerableValueType<T>();
            }
            else
            {
                result = reader.ToEnumerable<T>();
            }
        }
    }
    return result;

事实证明，SubSonic ToEnumerable 试图将数据读取器中的列名称与您要投影到的对象中的属性相匹配。来 self 的 Linq 的 SQL 查询如下所示:

SELECT [t0].[Id], [t0].[ProductId], [t0].[ReleaseDate], [t0].[ReleasedBy], [t0].[ReleaseNumber], [t0].[RevisionNumber], [t0].[c0]
FROM (
  SELECT [t1].[Id], [t1].[ProductId], [t1].[ReleaseDate], [t1].[ReleasedBy], [t1].[ReleaseNumber], [t1].[RevisionNumber], (
    SELECT COUNT(*)
    FROM [dbo].[Install] AS t2
    WHERE ([t2].[ReleaseId] = [t1].[Id])
    ) AS c0
  FROM [dbo].[Release] AS t1
  ) AS t0
WHERE ([t0].[ProductId] = 2)

请注意 [t0].[c0] 与我的属性名称 NumberOfInstalls 不同。所以 c0 的值永远不会转换到我的对象中。

修复:您只需删除 if 语句并使用慢 10 倍的投影，一切都会正常进行。