EF Core 8种子数据报错：无法创建DbContext类型实例

问题场景 #

执行迁移命令：

dotnet ef migrations add InitDb -s .\DemoProject.API\ -p .\DemoProject.EntityFramework\

触发以下错误：

无法创建''类型的'DbContext'。创建实例时抛出异常：'Character实体类型的种子实体无法添加，因为它已设置导航属性"Player"。要播种关系，请将实体种子添加到"Character"并指定外键值{'PlayerId'}。
考虑使用'DbContextOptionsBuilder.EnableSensitiveDataLogging'查看相关属性值。'

已在DataSeed.cs中为Character实体指定外键PlayerId，但问题仍存在。

相关代码 #

Character.cs #

public class Character
{
    public Character()
    {
        DateCreated = DateOnly.FromDateTime(DateTime.Now);
    }

    public Guid Id { get; set; }
    public string NickName { get; set; } = string.Empty;
    public string Classes { get; set; }
    public int Level { get; set; }
    public DateOnly DateCreated { get; set; }

    public Guid PlayerId { get; set; }
    public Player Player { get; set; }
}

CharacterMapping.cs #

public class CharacterMapping : IEntityTypeConfiguration<Character>
{
    public void Configure(EntityTypeBuilder<Character> builder)
    {
        builder.Property(x => x.NickName).HasMaxLength(20);
        builder.Property(x => x.Classes).HasMaxLength(20);
        builder.HasIndex(x => x.NickName).IsUnique();

        builder.HasOne(c => c.Player)
            .WithMany(p => p.Characters)
            .HasForeignKey(c => c.PlayerId);
    }
}

Player.cs #

public class Player
{
    public Player()
    {
        DateCreated = DateOnly.FromDateTime(DateTime.Now);
    }

    public Guid Id { get; set; }
    public string Account { get; set; }
    public string AccountType { get; set; }
    public DateOnly DateCreated { get; set; }
    public ICollection<Character> Characters { get; set; }
}

PlayerMapping.cs #

public class PlayerMapping:IEntityTypeConfiguration<Player>
{
    public void Configure(EntityTypeBuilder<Player> builder)
    {
        builder.Property(x => x.Account).HasMaxLength(50);
        builder.Property(x => x.AccountType).HasMaxLength(10);
        builder.HasIndex(x => x.Account).IsUnique();
    }
}

AppDbContext.cs #

public class AppDbContext:DbContext
{
    public AppDbContext(DbContextOptions options):base(options)
    {
    }

    public DbSet<Player> Players { get; set; }
    public DbSet<Character> Characters { get; set; }
    
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfiguration(new CharacterMapping());
        modelBuilder.ApplyConfiguration(new PlayerMapping());
        modelBuilder.Entity<Player>()
            .HasData(DataSeed.Players);
        modelBuilder.Entity<Character>()
            .HasData(DataSeed.Characters);
    }
}

DataSeed.cs #

public static class DataSeed
{
    public static List<Player> Players { get; } =
    [
        new Player
        {
            Id = Guid.NewGuid(),
            Account = "player1@example.com",
            AccountType = "Standard"
        },
        new Player
        {
            Id = Guid.NewGuid(),
            Account = "player2@example.com",
            AccountType = "Premium"
        },
        new Player
        {
            Id = Guid.NewGuid(),
            Account = "player3@example.com",
            AccountType = "Standard"
        }
    ];

    public static Character[] Characters { get; } = [
        new Character
        {
            Id = Guid.NewGuid(),
            NickName = "Character1",
            Classes = "Warrior",
            Level = 10,
            PlayerId = Players[0].Id
        },
        new Character
        {
            Id = Guid.NewGuid(),
            NickName = "Character2",
            Classes = "Mage",
            Level = 8,
            PlayerId = Players[0].Id
        },
        new Character
        {
            Id = Guid.NewGuid(),
            NickName = "Character3",
            Classes = "Ranger",
            Level = 12,
            PlayerId = Players[1].Id
        },
        new Character
        {
            Id = Guid.NewGuid(),
            NickName = "Character4",
            Classes = "Paladin",
            Level = 7,
            PlayerId = Players[2].Id
        },
        new Character
        {
            Id = Guid.NewGuid(),
            NickName = "Character5",
            Classes = "Druid",
            Level = 5,
            PlayerId = Players[2].Id
        }
    ];
}

问题根源 #

EF Core 种子数据要求实体的主键必须是固定不变的值，而你在DataSeed.cs中使用Guid.NewGuid()动态生成主键和外键。每次运行程序（包括生成迁移）时，这些Guid会重新生成，导致EF Core无法匹配Character的外键PlayerId与Player的主键Id之间的对应关系，进而误判为导航属性被非法设置。

解决步骤 #

1. 替换动态Guid为固定值：手动生成固定的Guid作为Player和Character的主键，确保每次程序运行时这些值都一致。
   修改后的DataSeed.cs示例：

   public static class DataSeed
   {
       // 固定Player主键
       private static readonly Guid Player1Id = new Guid("11111111-1111-1111-1111-111111111111");
       private static readonly Guid Player2Id = new Guid("22222222-2222-2222-2222-222222222222");
       private static readonly Guid Player3Id = new Guid("33333333-3333-3333-3333-333333333333");
   
       public static List<Player> Players { get; } =
       [
           new Player
           {
               Id = Player1Id,
               Account = "player1@example.com",
               AccountType = "Standard"
           },
           new Player
           {
               Id = Player2Id,
               Account = "player2@example.com",
               AccountType = "Premium"
           },
           new Player
           {
               Id = Player3Id,
               Account = "player3@example.com",
               AccountType = "Standard"
           }
       ];
   
       public static Character[] Characters { get; } = [
           new Character
           {
               Id = new Guid("10000000-0000-0000-0000-000000000000"),
               NickName = "Character1",
               Classes = "Warrior",
               Level = 10,
               PlayerId = Player1Id
           },
           new Character
           {
               Id = new Guid("10000000-0000-0000-0000-000000000001"),
               NickName = "Character2",
               Classes = "Mage",
               Level = 8,
               PlayerId = Player1Id
           },
           new Character
           {
               Id = new Guid("10000000-0000-0000-0000-000000000002"),
               NickName = "Character3",
               Classes = "Ranger",
               Level = 12,
               PlayerId = Player2Id
           },
           new Character
           {
               Id = new Guid("10000000-0000-0000-0000-000000000003"),
               NickName = "Character4",
               Classes = "Paladin",
               Level = 7,
               PlayerId = Player3Id
           },
           new Character
           {
               Id = new Guid("10000000-0000-0000-0000-000000000004"),
               NickName = "Character5",
               Classes = "Druid",
               Level = 5,
               PlayerId = Player3Id
           }
       ];
   }
   

2. 初始化Player的Characters集合（可选但推荐）：在Player的构造函数中初始化Characters集合，避免空引用问题：

   public Player()
   {
       DateCreated = DateOnly.FromDateTime(DateTime.Now);
       Characters = new List<Character>();
   }
   

3. 重新生成迁移：删除之前生成的迁移文件（如果有的话），重新执行迁移命令即可。

内容的提问来源于stack exchange，提问作者Kevin