c# - 对 Linq 和分组感到困惑

Question

我正在使用 Entity Framework 和 Linq，并尝试学习一些新东西。现在我有一个问题，我可以用 SQL 轻松解决，但想用 Linq 来解决。

我有一个颜色类和一个汽车类。每个 Car 对象都有一种颜色。

public class Color
{
    public int ColorID { get; set; }
    public string Name { get; set; }
}

public class Car
{
    public Guid CarID { get; set; }
    public string ModelName { get; set; }

    public int ColorID { get; set; }

    public virtual Color CarColor { get; set; }
}

现在我想创建一个查询来获取所有颜色(ID 和名称)以及使用该颜色的汽车数量(如果有的话)。

这次尝试给了我几乎正确的结果，但它排除了未使用的颜色。

        var n3 = from car in db.Cars
                 join c in db.Colors on car.ColorID equals c.ColorID into j1
                 from j2 in j1.DefaultIfEmpty()
                 group j2 by j2.ColorID into colorgroup
                 select new ColorCarCount
                 {
                     CarCount = colorgroup.Count(t => t.ColorID != null),
                     ColorID = colorgroup.Key, //colorgroup.Key,
                     Name = colorgroup.FirstOrDefault().Name
                 };

这种尝试感觉最好，但是这里我获取不到颜色名称:

        var n4 = from c in db.Colors
                 join car in db.Cars on c.ColorID equals car.ColorID into j1
                 from j2 in j1.DefaultIfEmpty()
                 group j2 by j2.ColorID into colorgroup
                 select new ColorCarCount
                 {
                     CarCount = colorgroup.Count(t => t.ColorID != null),
                     ColorID = colorgroup.Key, 
                     Name = "Can't get color name here"
                 };

有什么建议吗？是的，我读过类似的问题，但我不太明白 - 但我相信这很容易:-)

Answer 1

    var n4 = from c in db.Colors
             join car in db.Cars on c.ColorID equals car.ColorID into tmp
             from car in tmp.DefaultIfEmpty()
             group car by c into colorgroup
             select new ColorCarCount
             {
                 CarCount = colorgroup.Count(),
                 ColorID = colorgroup.Key.ColorID,
                 Name = colorgroup.Key.Name
             };