需要使用扩展方法/查询语法在LINQ中进行左外部联接

Question

i have a schema with the following structure;

Basically, a menu can have nested childs. And the other tables just holds ItemType , and the Id of the that item ie ItemTypeId (these are referenced dynamically when user choos options at backend). Now, for menus to be displayed, i have used Inner Join (i want here to use Left Outer Join ) here;

public IQueryable<Menu> GetMenuWithAsset()
{
    return DbContext.Set<Menu>()
                    .Join(DbContext.Set<Asset>(), m => m.MenuId, a => a.MenuId, (m, a) => m);
}

and in my base controller, i'm fetching it like this;

protected override void OnActionExecuting(ActionExecutingContext filterContext)
{
    var menus = Uow.Menus.GetMenuWithAsset()

        .Select(m => new ParentMenuViewModel()
        {
            MenuId = m.MenuId,
            Name = m.Name,
            ParentId = m.ParentId
        })
        .ToList<ParentMenuViewModel>();
    ViewBag.Menus = CreateVM(0, menus);
    base.OnActionExecuting(filterContext);
}

public IEnumerable<ParentMenuViewModel> CreateVM(int parentid, List<ParentMenuViewModel> list)
{
    var newList = list.Where(m=> m.ParentId==parentid)
        .Select(m=> new ParentMenuViewModel()
        {
            MenuId = m.MenuId,
            Name = m.Name,
            ParentId = m.ParentId,
            ChildMenus = CreateVM(m.MenuId, list)
        });
    return newList;
}

And this code works correctly as expected. However, here is what i want;
1) I want to display all the menus(including child menus) whetere we have data or not in Asset table ( i gess left outer join).
2) I want to achieve this using strongly typed viewmodel which will hold the following properties from both of the tables
i) MenuId ii) Name iii)ParentId iv) ChildMenus v)MenuItemType vi) MenuItemTypeId
I'm trying to solve this from 4 days. Any help regard to this is apreciatable;

Answer 1

Try this query:

DbContext.Set<Menu>()
         .GroupJoin(DbContext.Set<Asset>(), m => m.MenuId, a => a.MenuId, (m, as) => new { m, as = as.DefaultIfEmpty() });

It will return anonymus class with property m of type Menu and property as that contains IQueriable of Assets. I hope this is what you want.