Question

我有两个实体-团队和游戏。一个团队可以有很多场比赛（一对多）。

所以看起来像这样：

The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP or FOR XML is also specified.

这很好用，但我想通过将游戏分为两类（主场和客场游戏）来使其更加精致。但是，这将在两个实体之间引入另一个关系，我不确定如何定义它。

我想会是这样吗？

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

        public ICollection<Game> Games { get; set; }
    }

 public class Game
    {
        public int Id { get; set; }
        public DateTime Date { get; set; }

        public int TeamId { get; set; }
        public Team Team { get; set; }
    }

这样做会混淆实体框架，并且无法决定如何解决关系。

有什么想法吗？

Answer 1

基于Relationships - EF Core | Microsoft Docs，您可以使用数据注释

数据注释

有两个数据注释可用于配置   关系，[ForeignKey]和[InverseProperty]。

[ForeignKey]

您可以使用数据注释来配置   属性应用作给定的外键属性   关系。通常在外键属性为   未被惯例发现。

[InverseProperty]

您可以使用数据注释来配置   从属实体和主体实体上的导航属性配对。   通常在导航对多于一对时完成   两种实体类型之间的属性。

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

        [InverseProperty("HomeTeam")]
        public ICollection<Game> HomeGames { get; set; }

        [InverseProperty("AwayTeam")]
        public ICollection<Game> AwayGames { get; set; }
    }

public class Game
    {
        public int Id { get; set; }
        public DateTime Date { get; set; }

        public int HomeTeamId { get; set; }
        [ForeignKey("HomeTeamId")]
        public Team HomeTeam { get; set; }

        public int AwayTeamId{ get; set; }
        [ForeignKey("AwayTeamId")]
        public virtual Team AwayTeam { get; set; }
    }

如果使用db.Database.Migrate（），则会出现错误提示

System.Data.SqlClient.SqlException：'引入外键表“游戏”上的约束“ FK_Games_Teams_HomeTeamId”可能会导致循环或多个级联路径。指定ON DELETE NO ACTION或ON UPDATE NO ACTION，或修改其他FOREIGN KEY约束。不能创建约束或索引。查看先前的错误

您可以将HomeTeamId AwayTeamId int？设置为可空

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

        [InverseProperty("HomeTeam")]
        public ICollection<Game> HomeGames { get; set; }

        [InverseProperty("AwayTeam")]
        public ICollection<Game> AwayGames { get; set; }
    }

public class Game
    {
        public int Id { get; set; }
        public DateTime Date { get; set; }

        public int? HomeTeamId { get; set; }
        [ForeignKey("HomeTeamId")]
        public Team HomeTeam { get; set; }

        public int? AwayTeamId{ get; set; }
        [ForeignKey("AwayTeamId")]
        public virtual Team AwayTeam { get; set; }
    }

或参见Cascade Delete - EF Core | Microsoft Docs

这里是我测试和工作的完整代码（先而不是先编码）
对于代码，请先使用 int？

对于Program.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using Microsoft.EntityFrameworkCore;

namespace stackoverflow54196199
{

public class Team
{

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

    [InverseProperty("HomeTeam")]
    public ICollection<Game> HomeGames { get; set; }

    [InverseProperty("AwayTeam")]
    public ICollection<Game> AwayGames { get; set; }
}

public class Game
{
    public int Id { get; set; }
    public DateTime Date { get; set; }

    public int HomeTeamId { get; set; }
    [ForeignKey("HomeTeamId")]
    public Team HomeTeam { get; set; }

    public int AwayTeamId { get; set; }
    [ForeignKey("AwayTeamId")]
    public Team AwayTeam { get; set; }
}


public class MyContext : DbContext
{
    public DbSet<Game> Games { get; set; }
    public DbSet<Team> Teams { get; set; }


    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=.;Integrated Security=true;Initial Catalog=stackoverflow54196199;Persist Security Info=False;");
    }
}

class Program
{
    static void Main(string[] args)
    {
        var db = new MyContext();
        foreach (var game in db.Games.Include(i => i.AwayTeam).Include(i => i.HomeTeam))
        {
            Console.WriteLine(game.HomeTeam.Name);
            Console.WriteLine(game.AwayTeam.Name);

        }
        Console.ReadLine();
    }
}
}

对于stackoverflow54196199.csproj

<PropertyGroup>
  <OutputType>Exe</OutputType>
  <TargetFramework>netcoreapp2.1</TargetFramework>
</PropertyGroup>

<ItemGroup>
  <PackageReference Include="Microsoft.EntityFrameworkCore" Version="2.1.0" />
  <PackageReference Include="Microsoft.EntityFrameworkCore.Relational" Version="2.1.0" />
  <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="2.1.0" />
 </ItemGroup>

Answer 2

您必须告诉Entity Framework一个关联中涉及两个实体中的哪些属性。在流利的Mapping API中，这是：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Team>().HasMany(t => t.HomeGames)
        .WithOne(g => g.HomeTeam)
        .HasForeignKey(g => g.HomeTeamId);
    modelBuilder.Entity<Team>().HasMany(t => t.AwayGames)
        .WithOne(g => g.AwayTeam)
        .HasForeignKey(g => g.AwayTeamId).OnDelete(DeleteBehavior.Restrict);
}

您必须使用fluent API，因为默认情况下，EF将尝试使用级联删除创建两个外键。由于臭名昭著的“多个级联路径”限制，SQL Server不允许这样做。密钥之一不应该是级联的，只能由流利的API配置。

实体框架核心-两个实体之间的多个一对多关系

2 个答案: