c# - 复合键作为外键

Question

我在 MVC 3 应用程序中使用实体框架 4.1。我有一个实体，我的主键由两列（复合键）组成。这在另一个实体中用作外键。如何建立关系？在正常的 scnerios 中，我们使用：

public class Category
{
    public string CategoryId { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Product> Products { get; set; }
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public string CategoryId { get; set; }

    public virtual Category Category { get; set; }
} 

但是如果类别有两列键怎么办？

Answer 1

您可以使用任一流畅的 API：

public class Category
{
    public int CategoryId1 { get; set; }
    public int CategoryId2 { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Product> Products { get; set; }
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public int CategoryId1 { get; set; }
    public int CategoryId2 { get; set; }

    public virtual Category Category { get; set; }
}

public class Context : DbContext
{
    public DbSet<Category> Categories { get; set; }
    public DbSet<Product> Products { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Category>()
            .HasKey(c => new {c.CategoryId1, c.CategoryId2});

        modelBuilder.Entity<Product>()
            .HasRequired(p => p.Category)
            .WithMany(c => c.Products)
            .HasForeignKey(p => new {p.CategoryId1, p.CategoryId2});

    }
}

或数据注释：

public class Category
{
    [Key, Column(Order = 0)]
    public int CategoryId2 { get; set; }
    [Key, Column(Order = 1)]
    public int CategoryId3 { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Product> Products { get; set; }
}

public class Product
{
    [Key]
    public int ProductId { get; set; }
    public string Name { get; set; }
    [ForeignKey("Category"), Column(Order = 0)]
    public int CategoryId2 { get; set; }
    [ForeignKey("Category"), Column(Order = 1)]
    public int CategoryId3 { get; set; }

    public virtual Category Category { get; set; }
}

Answer 2

我相信最简单的方法是在 Navigation 属性上使用 Data Annotation，如下所示： [ForeignKey("CategoryId1, CategoryId2")]

public class Category
{
    [Key, Column(Order = 0)]
    public int CategoryId1 { get; set; }
    [Key, Column(Order = 1)]
    public int CategoryId2 { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Product> Products { get; set; }
}

public class Product
{
    [Key]
    public int ProductId { get; set; }
    public string Name { get; set; }
    public int CategoryId1 { get; set; }
    public int CategoryId2 { get; set; }

    [ForeignKey("CategoryId1, CategoryId2")]
    public virtual Category Category { get; set; }
}

Answer 3

在.NET Core和.NET 5 <文档中只显示Data annotations (simple key).

https://docs.microsoft.com/en-us/ef/core/modeling/relationships?tabs=fluent-api%2Cfluent-api-composite-key%2Csimple-key#foreign-key

但是，使用@LadislavMrnka 中的示例，您将收到如下错误消息：

System.InvalidOperationException：[ForeignKey] 属性指向导航“”的多个属性。要使用数据注释定义复合外键，请使用导航上的 [ForeignKey] 属性。

使用该错误消息，您可以编写如下代码：

public class Product
{
    [Key]
    public int ProductId { get; set; }
    public string Name { get; set; }

    public int CategoryId2 { get; set; }

    public int CategoryId3 { get; set; }

    [ForeignKey("CategoryId2,CategoryId3")]
    public virtual Category Category { get; set; }
}

Fluent API (composite key)来自微软的例子：

internal class MyContext : DbContext
{
    public DbSet<Car> Cars { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Car>()
            .HasKey(c => new { c.State, c.LicensePlate });

        modelBuilder.Entity<RecordOfSale>()
            .HasOne(s => s.Car)
            .WithMany(c => c.SaleHistory)
            .HasForeignKey(s => new { s.CarState, s.CarLicensePlate });
    }
}

public class Car
{
    public string State { get; set; }
    public string LicensePlate { get; set; }
    public string Make { get; set; }
    public string Model { get; set; }

    public List<RecordOfSale> SaleHistory { get; set; }
}

public class RecordOfSale
{
    public int RecordOfSaleId { get; set; }
    public DateTime DateSold { get; set; }
    public decimal Price { get; set; }

    public string CarState { get; set; }
    public string CarLicensePlate { get; set; }
    public Car Car { get; set; }
}