子表上的 EF Core 联接

Question

我设置了三个类，如下所示：

public class Item
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<UserItem> UserItems { get; set; }
}

public class User
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<UserItem> UserItems { get; set; }
}

public class UserItem
{
    public int ID { get; set; }
    public int UserID { get; set; }
    public int ItemID { get; set; }
}

Entity Framework 使用正确的外键在 SQL Server 数据库中生成表。我可以像这样查询Item 和User 以获取UserItem 集合：

var a = context.DBUser.Include(s => s.UserItems);
var b = context.DBItem.Include(s => s.UserItems);

我不满意的是我如何从User 到Item。在 SQL 中，我会做这样的事情

SELECT * FROM Users U
LEFT JOIN UserItems UI
    ON U.ID = UI.UserID
LEFT JOIN Items I
    ON UI.ItemID = I.ID

在 C# 中，我需要求助于这个：

var c = from user in _context.DBUsers
    join userItem in _context.DBUserItems
        on user.ID equals userItem.UserID into ui
        from userItem in ui.DefaultIfEmpty()
    join item in _context.DBItems
        on userItem.ItemID equals item.ID into i
        from item in i.DefaultIfEmpty()
    select new
    {
        user,
        userItem,
        item
    };

鉴于 EF 中的表之间应该已经存在关系感觉不对

Answer 1

您还需要向UserItem 添加导航：

public class UserItem
{
    public int ID { get; set; }
    public int UserID { get; set; }
    public int ItemID { get; set; }

    public User User { get; set; }
    public Item Item { get; set; }
}

那么你的查询可以是这样的：

var users = context.DBUser
    .Include(x => x.UserItems)
    .ThenInclude(x => x.Item);

另请注意，如果您使用的是 .NET 5，则可以完全省略 UserItem“连接实体”：

public class Item
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<User> Users { get; set; }
}

public class User
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Item> Items { get; set; }
}

这将简化您的查询：

var users = context.DBUser.Include(x => x.Items);