如果 EF Core 5 以多对多关系播种数据

Question

我有用户实体

public class User
{
    public int UserId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }

    public ICollection<Technology> Technologies { get; set; } = new List<Technology>();
}

我有技术实体

public class Technology
{
    public int TechnologyId { get; set; }
    public string TitleTechnology { get; set; }
    public int GroupId { get; set; }
    public Group Group { get; set; }
    public ICollection<User> Users { get; set; } = new List<User>();
}

我想创建多对多关系，所以我有这样的OnModelCreating 方法：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    var groupList = new List<Group>
    {
        new Group {GroupId = 1, TitleGroup = ".NET"}
    };

    var technologyList = new List<Technology>
    {
        new Technology {TechnologyId = 1, GroupId = 1, TitleTechnology = ".NET 5"},
        new Technology {TechnologyId = 2, GroupId = 1, TitleTechnology = ".NET Framework 4.8"},
        new Technology {TechnologyId = 3, GroupId = 1, TitleTechnology = "EF 6"},
        new Technology {TechnologyId = 4, GroupId = 1, TitleTechnology = "ASP.NET MVC 5"}
    };

    var userList = new List<User>
    {
        new User
        {
            UserId = 1, FirstName = "Serhii", LastName = "Yurko", Email = "test", Password = "test",
            Technologies = new List<Technology> {technologyList[0], technologyList[1]}
        }
    };

    modelBuilder.Entity<Technology>().HasOne(exp => exp.Group).WithMany(exp => exp.Technologies).HasForeignKey(exp => exp.GroupId);
    modelBuilder.Entity<User>().HasMany(p => p.Technologies).WithMany(p => p.Users)
        .UsingEntity(j => j.ToTable("UserTechnology"));

    modelBuilder.Entity<User>().HasData(userList);
    modelBuilder.Entity<Group>().HasData(groupList);
    modelBuilder.Entity<Technology>().HasData(technologyList);
    base.OnModelCreating(modelBuilder);
}

当我想创建迁移时，我会收到这样的异常 -

无法添加实体类型“用户”的种子实体，因为它有 导航“技术”集。要播种关系，请添加 'TechnologyUser (Dictionary)' 的实体种子和 指定外键值 {'UsersUserId'}。考虑使用 'DbContextOptionsBuilder.EnableSensitiveDataLogging' 查看 涉及属性值。

如何建立适当的关系？

Answer 1

在 EF-core 中，您不能直接播种导航属性。您必须从 User 初始化中删除行 Technologies = ...。

如here 所述，联结表可以通过扩展UsingEntity() 调用由其自己的HasData 调用播种，如下所示：

.UsingEntity(j => j
    .ToTable("UserTechnology")
    .HasData(new[]
        {
            { UsersID = 1, TechnologiesID = 1 },
            { UsersID = 1, TechnologiesID = 2 }
        }
    ));

如您所见，隐藏的联结实体由具有与表字段相同的属性名称（+ 类型）的匿名类型填充。这需要你知道这些外键字段的命名约定（我希望我猜对了）。

如documentation 和this Stack Overflow answer 中所述，您可以使用连接表的相当笨重的完全初始化来完成这一切。这允许您强制执行自己的 FK 属性名称并在种子代码中相应地使用它们。

Answer 2

Model seed data 方法有一些已知的限制。最值得注意的是，它不能通过从导航属性推断关系来插入相关数据，您必须显式添加外键值。详情见-Limitations of model seed data

在您的情况下，最好使用Custom initialization logic 方法，如此处所述 - Custom initialization logic（不要忘记仔细阅读Warning）。

Custom initialization logic 方法的实现：
对于模型 -

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }

    public ICollection<Technology> Technologies { get; set; } = new List<Technology>();
}

public class Technology
{
    public int Id { get; set; }
    public string TitleTechnology { get; set; }
    public int GroupId { get; set; }

    public Group Group { get; set; }
    public ICollection<User> Users { get; set; } = new List<User>();
}

public class Group
{
    public int Id { get; set; }
    public string TitleGroup { get; set; }

    public ICollection<Technology> Technologies { get; set; } = new List<Technology>();
}

配置-

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Technology>()
        .HasOne(exp => exp.Group)
        .WithMany(exp => exp.Technologies)
        .HasForeignKey(exp => exp.GroupId);

    modelBuilder.Entity<User>()
        .HasMany(p => p.Technologies)
        .WithMany(p => p.Users);
}

一个可能的实现可能看起来像 -

private static async Task AddEntities(DbCtx dbCtx)
{
    List<Group> groupList;
    if (!dbCtx.Groups.Any())
    {
        groupList = new List<Group>
        {
            new Group { TitleGroup = ".NET"}
        };
        dbCtx.Groups.AddRange(groupList);
    }
    else
    {
        groupList = dbCtx.Groups.ToList();
    }

    List<Technology> technologyList;
    if (!dbCtx.Technologies.Any())
    {
        technologyList = new List<Technology>
        {
            new Technology { Group = groupList[0], TitleTechnology = ".NET 5"},
            new Technology { Group = groupList[0], TitleTechnology = ".NET Framework 4.8"},
            new Technology { Group = groupList[0], TitleTechnology = "EF 6"},
            new Technology { Group = groupList[0], TitleTechnology = "ASP.NET MVC 5"}
        };
        dbCtx.Technologies.AddRange(technologyList);
    }
    else
    {
        technologyList = dbCtx.Technologies.ToList();
    }

    List<User> userList;
    if (!dbCtx.Users.Any())
    {
        userList = new List<User>
        {
            new User
            {
                Name = "Serhii", Technologies = new List<Technology> { technologyList[0], technologyList[1]}
            }
        };
        dbCtx.Users.AddRange(userList);
    }

    await dbCtx.SaveChangesAsync();
}

这取决于您在哪里以及如何使用它。我自己通过IHost 扩展方法使用它，例如 -

public static async Task GenerateSeedDataAsync(this IHost host)
{
    using (var scope = host.Services.CreateScope())
    {
        var provider = scope.ServiceProvider;
        var dbCtx = provider.GetRequiredService<DbCtx>();
        dbCtx.Database.Migrate();

        await AddEntities(dbCtx);
    }
}

这样每次应用启动时，我们都可以选择检查数据库是否需要做种，比如-

public static void Main(string[] args)
{
    IHost host = CreateHostBuilder(args).Build();
    host.GenerateSeedDataAsync().Wait();
    host.Run();
}