C# LINQ Union列（如果已验证）

我有一个简单的项目评级应用程序。有三个主要表格：

Item
{
   ItemID,
   Contents
}

asp_User
{
   UserID,
   Name
}

Rating
{  
   RatingID,
   ItemID,
   UserID,
   int Rating
}

我有linq代码来读取项目：

var q = from item db.Item
        select item;

然后，我想在q后面附加一列，其中包含当前已验证用户的每个项目行的评级。如果用户未登录，或者认证用户未提供评级，则结果将为0

我正在使用SqlMembershipProvider，如果这很重要的话

范例

q的最终结果应该如下所示：

[已验证]

//The currently authenticated user has commented on Item.ID = 1 and has given it a 9.
q = {ID = 1, Contents = "Whatever", Rating = 9},

//The currently Authenticated user has not commented on Item.ID = 2.
{ID = 2, Contents = "Something", Rating = 0};

//There is not an authenticated user
q = {ID = 1, Contents = "Whatever", Rating = 0},

//There is not an authenticated user
{ID = 2, Contents = "Something", Rating = 0};

[未经认证]

//The currently authenticated user has commented on Item.ID = 1 and has given it a 9.
q = {ID = 1, Contents = "Whatever", Rating = 9},

//The currently Authenticated user has not commented on Item.ID = 2.
{ID = 2, Contents = "Something", Rating = 0};

//There is not an authenticated user
q = {ID = 1, Contents = "Whatever", Rating = 0},

//There is not an authenticated user
{ID = 2, Contents = "Something", Rating = 0};

---

您打算如何处理“var q”？很难从这个问题中看出你的意图，在这种情况下，这很重要。基于缺乏背景，以下是我能做的最好的。希望这将有助于：

if(User.IsAuthenticated)
{
    var q = from item in db.Item
            join r in db.Rating on c.Id equals r.ItemId
            select new {ID = item.Id, Contents = item.Contents, Rating = r.Rating};

    // Do whatever you want with this here.
}
else
{
    var y = db.Item;

    // Do whatever you want with this here.
}

---

从性能角度来看，解决方案是可行的（尤其是在查询多个项目时）。 但为了满足评级应该是Item属性的条件，可以通过在LINQ-to-SQL数据类的同一命名空间中使用分部类来扩展Item类：

public partial class Item
{
    public int Rating
    {
        get
        {
            if (!Thread.CurrentPrincipal.Identity.IsAuthenticated)
                return 0;

            using (var db = new ApplicationDataContext())
            {
                return db.Item
                    .Where(r => r.ItemID == this.ItemID
                        && r.UserID == Thread.CurrentPrincipal.Identity.Name)
                    .Select(r => r.Rating)
                    .SingleOrDefault();
            }
        }
    }
}

如果您想通过像Ocelot proposed这样的单个SQL请求获得相同的结果，那么您应该使用GroupJoin（类似于SQL LEFT外部联接），并像我上面所做的那样选择当前用户评级