问题描述
我设置了三个类:
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。我可以查询 Item
和 User
以获取 UserItem
集合,如下所示:
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,这感觉不对
解决方法
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
“加入实体”:
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);