C# DbContext返回一些空字段

Q:当我尝试从

\u Db

返回数据时，我在

列表

属性中得到null对象，我想知道为什么其他属性返回正确，而

列表

没有

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

问题解释：我有一个

ApplicationDbContext

类

公共IDbSet播放器{get；set；}

属性。使用以下

ApplicationBinInitializer

类：

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

我对数据的请求如下所示：

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

public class Player
{
    [Key]
    public int Id { get; set; }
    public string PlayerName { get; set; }
    public List<string> PlayerSkills { get; set; }
 }

public IEnumerable<Player> Get()
{
    return _Db.Players;
}

---

SQL中有一种类型的字段可以存储值集合。若你们看一下EF生成的表格，你们将不会看到

PlayerSkills

归档在那个里。创建脚本将如下所示

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

CREATE TABLE [dbo].[Players] (
    [Id] [int] NOT NULL IDENTITY,
    [PlayerName] [nvarchar](max),
    CONSTRAINT [PK_dbo.Players] PRIMARY KEY ([Id])
)

这就是为什么在

PlayerSkills

字段中没有任何数据。若要在SQL数据库中存储一对多关系，则需要第二个表来存储玩家技能以及这些技能相关的玩家id。如果您想避免技能重复，甚至可以使用两个表—一个用于技能，另一个用于将玩家映射到技能

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

---

没有连接表（如果您不希望外键具有显式属性，那么您的选项是fluent映射而不是属性映射）：

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

玩家

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

public class Player
{
    public int Id { get; set; }
    public string PlayerName { get; set; }
    public virtual List<PlayerSkill> PlayerSkills { get; set; }
}

公共类播放器
{
公共int Id{get；set；}
公共字符串播放器名称{get；set；}
公共虚拟列表玩家技能{get；set；}
}

---

带连接表

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Player>()
        .HasMany(p => p.PlayerSkills)
        .WithMany()
        .Map(j => j.MapLeftKey("PlayerId")
                   .MapRightKey("PlayerSkillId")
                   .ToTable("PlayerToSkill"));

    base.OnModelCreating(modelBuilder);
}

模型创建时受保护的覆盖无效（DbModelBuilder modelBuilder）
{
modelBuilder.Entity（）
.HasMany（p=>p.PlayerSkills）
.有很多
.Map（j=>j.MapLeftKey（“PlayerId”）
.MapRightKey（“PlayerSkillId”）
.ToTable（“PlayerToSkill”）；
基于模型创建（modelBuilder）；
}

SQL中有一种类型的字段可以存储值集合。若你们看一下EF生成的表格，你们将不会看到

PlayerSkills

归档在那个里。创建脚本将如下所示

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

CREATE TABLE [dbo].[Players] (
    [Id] [int] NOT NULL IDENTITY,
    [PlayerName] [nvarchar](max),
    CONSTRAINT [PK_dbo.Players] PRIMARY KEY ([Id])
)

这就是为什么在

PlayerSkills

字段中没有任何数据。若要在SQL数据库中存储一对多关系，则需要第二个表来存储玩家技能以及这些技能相关的玩家id。如果您想避免技能重复，甚至可以使用两个表—一个用于技能，另一个用于将玩家映射到技能

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

---

没有连接表（如果您不希望外键具有显式属性，那么您的选项是fluent映射而不是属性映射）：

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

玩家

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

public class Player
{
    public int Id { get; set; }
    public string PlayerName { get; set; }
    public virtual List<PlayerSkill> PlayerSkills { get; set; }
}

公共类播放器
{
公共int Id{get；set；}
公共字符串播放器名称{get；set；}
公共虚拟列表玩家技能{get；set；}
}

---

带连接表

public class ApplicationDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        context.Players.Add(new Player { PlayerName = "john", PlayerSkills = new List<string> { "a", "b" } });
        context.Players.Add(new Player { PlayerName = "Wuli", PlayerSkills = new List<string> { "c", "d" } });

        base.Seed(context);
    }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Player>()
        .HasMany(p => p.PlayerSkills)
        .WithMany()
        .Map(j => j.MapLeftKey("PlayerId")
                   .MapRightKey("PlayerSkillId")
                   .ToTable("PlayerToSkill"));

    base.OnModelCreating(modelBuilder);
}

模型创建时受保护的覆盖无效（DbModelBuilder modelBuilder）
{
modelBuilder.Entity（）
.HasMany（p=>p.PlayerSkills）
.有很多
.Map（j=>j.MapLeftKey（“PlayerId”）
.MapRightKey（“PlayerSkillId”）
.ToTable（“PlayerToSkill”）；
基于模型创建（modelBuilder）；
}

从SQL的角度来看，

PlayerSkills

需要存储字符串集合的字段类型是什么？好吧，就这样吧。。是代码优先、模型优先、数据库优先吗？如果首先是数据库——也许是。还有一点可能会影响事情。。它是实体核心吗？还是实体框架6？啊!！查看您的

公共列表玩家技能

我想知道您是否理解..@BagusTesa它是代码优先和实体框架6。是的，我可能错过了一些概念-第一次使用db。从SQL的角度来看，

PlayerSkills

必须存储字符串集合的字段类型是什么。。是代码优先、模型优先、数据库优先吗？如果首先是数据库——也许是。还有一点可能会影响事情。。它是实体核心吗？还是实体框架6？啊!！查看您的

公共列表玩家技能

我想知道您是否理解..@BagusTesa它是代码优先和实体框架6。是的，我可能错过了一些概念——第一次与db合作。