无法使用 codefirst 在多对多关系中保存联合表中的数据

Question

我首先是EntityFramework代码的新手。所以我正在运行一些简单的测试来学习。我创建了两个类：团队和玩家，假设它们具有多对多关系。我还启用了迁移并添加了一些种子数据。当我运行更新数据库时，我看到种子数据填充了球队和球员表，但队员表是空的。代码中有什么问题的任何线索？(为了便于演示，我在这里删除了包包含的内容(

namespace CodeFirst_onetomany.Models
{
public class Team
{
public Team()
{
this.Players = new HashSet<Player>();
}
public int Id { get; set; }
public string TeamName { get; set; }
public virtual ICollection<Player> Players { get; set; }
}
}
namespace CodeFirst_onetomany.Models
{
public class Player
{
public Player()
{
this.Teams = new HashSet<Team>();
}
public int Id { get; set; }
public string FirstName { get; set; }
[Required]
public string LastName { get; set; }
public virtual ICollection<Team> Teams { get; set; }
}
}
namespace CodeFirst_onetomany.Models
{
public class MyContext : DbContext
{
public MyContext() : base("MyDbOneToMany")
{
}
public DbSet<Team> Teams { get; set; }
public DbSet<Player> Players { get; set; }
}
}
namespace CodeFirst_onetomany.Migrations
{
using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Data.Entity.Migrations;
using System.Linq;
internal sealed class Configuration : DbMigrationsConfiguration<CodeFirst_onetomany.Models.MyContext>
{
public Configuration()
{
AutomaticMigrationsEnabled = false;
}
protected override void Seed(CodeFirst_onetomany.Models.MyContext context)
{
//  This method will be called after migrating to the latest version.
//  You can use the DbSet<T>.AddOrUpdate() helper extension method 
//  to avoid creating duplicate seed data. E.g.
//
//    context.People.AddOrUpdate(
//      p => p.FullName,
//      new Person { FullName = "Andrew Peters" },
//      new Person { FullName = "Brice Lambson" },
//      new Person { FullName = "Rowan Miller" }
//    );
//
context.Teams.AddOrUpdate(t => t.TeamName, new Team() { TeamName = "AC Milan" });
context.Teams.AddOrUpdate(t => t.TeamName, new Team() { TeamName = "Barcelona" });
context.SaveChanges();
context.Players.AddOrUpdate(p => new { p.FirstName, p.LastName },
new Player()
{
FirstName = "Paolo",
LastName = "Maldini",
Teams = new List<Team>() { context.Teams.FirstOrDefault(t => t.TeamName == "AC Milan") }
});
context.Players.AddOrUpdate(p => new { p.FirstName, p.LastName },
new Player()
{
FirstName = "Leo",
LastName = "Messi",
Teams = context.Teams.ToList()
});
context.SaveChanges();
}
}
}

编辑：所以我删除了 HashSets(感谢您提供有关该@Harald Coppoolse 的信息(并开始将信息存储在变量中，以便我可以调试它......我还在最后添加了两条额外的行，为玩家添加团队。所以我有部分：

context.Players.AddOrUpdate(p => new { p.FirstName, p.LastName }, maldini);
context.Players.AddOrUpdate(p => new { p.FirstName, p.LastName }, messi);
context.Players.FirstOrDefault(p => p.FirstName == "Paolo").Teams = shouldBeMilan;
context.Players.FirstOrDefault(p => p.FirstName == "Leo").Teams = shouldBeBarcaMilan;
context.SaveChanges();

它现在可以工作了。所以我想必须在 EF 中手动添加玩家和团队之间的关系，我们不能依赖对象创建(即使用 new(。我不知道为什么！

Answer 1

您是否尝试过在不使用迁移的情况下添加包含某些玩家的团队？

顺便说一下：尝试一下，不要在构造函数中创建 HashSet。这是对处理能力的浪费，因为它将立即被自己的 ICollection 所取代。

以下内容在没有哈希集的普通 Program.Main(( 中对我有用：

using (var dbContext = new MyDbContext(...)
{
var team1 = dbContext.Teams.Add(new Team() {TeamName = "Team 1"});
var team2 = dbContext.Teams.Add(new Team()
{
TeamName = "My Super Team",
Players =  new List<Player>()
{
new Player() {FirstName = "Christopholo", LastName = "Columbo"},
new Player() {FirstName = "Marco", LastName = "Polo"},
},
});
var player1 = dbContext.Players.Add(new Player()
{
FirstName = "X1",
LastName = "Y1",
});
var player2 = dbContext.Players.Add(new Player()
{
FirstName = "X2",
LastName = "Y2",
Teams = new List<Team>() {team1, team2, new Team() {TeamName = "Team3"});
});
dbContext.SaveChanges();
var teams = dbContext.Teams.Select(team => new
{
Id = team.Id,
Name = team.Name,
Players = team.Players.Select(player => new
{
Id = player.Id,
LastName = player.LastName,
})
.ToList(),
})
.ToList();
var players = dbContext.Players.Select(player => new
{
Id = player.Id,
LastName = player.LastName,
Teams = player.Teams.Select(team => new
{
Id = team.Id,
Name = team.TeamName,
})
.ToList(),
})
.ToList();
.ToList();
}

这行得通吗？如果你在这个团队中添加一名球员怎么办？

// add a Player using the Team's collection:
var teamToUpdate = dbContext.Teams.Where(team => team.Id ==team1.Id;
teamToUpdate.Players.Add(new Player() {FirstName = "...", LastName = "..."});
// add a Player and give him a Team:
var addedPlayer = dbContext.Player(new Player()
{
FirstName = ...,
LastName = ...,
Teams = new List<Team>() {teamToUpdate},
})
dbContext.SaveChanges();

如果您在使用AddOrUpdate时执行此操作会发生什么？

我已经尝试过这个(都没有哈希集(并且它可以工作。

建议：首先让它在没有迁移的情况下工作，然后尝试在迁移中执行此操作。使用断点调试它，您真的在迁移吗？