EF Core中能否抽象带附加数据的桥接实体？

我正在探索EF Core 8的设计空间，当前需要抽象EF Core中带附加数据的桥接实体的部分配置，但创建数据库迁移时遇到了错误，具体情况如下：

抽象桥接类定义 #

public abstract class Bridge<Q,R,S,T> where Q : Entity<R> where S : Entity<T>
{
    protected internal Q Left { get; set; }
    protected internal R LeftId { get; set; }
    protected internal S Right { get; set; }
    protected internal T RightId { get; set; }
}

具体桥接实体实现 #

public class TestBridgeEntity : Bridge<LeftSideEntity, Guid, RightSideEntity, Guid>
{
    public LeftSideEntity MyLeftSideEntity 
    {
        get { return Left; } 
        set { Left = value; } 
    }
    
    public Guid LeftSideEntityId 
    {
        get { return LeftId; } 
        set { LeftId = value; } 
    }

    public RightSideEntity MyRightSideEntity 
    {
        get { return Right; } 
        set { Right = value; } 
    }

    public Guid RightSideEntityId 
    { 
        get { return RightId; } 
        set { RightId = value; } 
    }

    public double MyTestData { get; set; }
}

尝试的配置扩展方法 #

public static EntityTypeBuilder<T> ConfigureBridge<T,A,B,C,D>(this EntityTypeBuilder<T> builder, string leftIdName, string rightIdName)
 where T : Bridge<A,B,C,D> where A: Entity<B> where C:Entity<D>
{
    builder.HasKey(x => new { x.LeftId, x.RightId } );

    builder.HasOne(x => x.Left)
           .WithMany()
           .HasForeignKey(x => x.LeftId);

    builder.HasOne(x => x.Right)
           .WithMany()
           .HasForeignKey(x => x.RightId);

    builder.Property(e => e.LeftId).HasColumnName(leftIdName);
    builder.Property(e => e.RightId).HasColumnName(rightIdName);

    return builder;
}

实体配置类 #

internal class MyTestBirdgeEntityConfiguration : IEntityTypeConfiguration<TestBridgeEntity>
{
    public void Configure(EntityTypeBuilder<TestBridgeEntity> builder)
    {
        builder.ToTable("test_bridge");
        builder.ConfigureBridge<TestBridgeEntity, LeftSideEntity, Guid, RightSideEntity, Guid>("left_side_id", "right_side_id");

        builder.Property(e => e.MyTestData ).IsRequired();
    }
}

关联实体定义 #

public class LeftSideEntity
{
    // ...

    public List<RightSideEntity> RightSideEntities { get; }
    public List<TestBridgeEntity> TestBridgeEntities { get; }

    // ...
}

RightSideEntity未引用LeftSideEntities集合。

迁移错误信息 #

Unable to create a 'DbContext' of type 'TestDbContext'. The exception ''TestBridgeEntity.LeftId' and 'TestBridgeEntity.LeftSideEntityId' are both mapped to column 'left_side_id' in 'test_bridge', but the properties are contained within the same hierarchy. All properties on an entity type must be mapped to unique different columns.'

解决方案 #

原因分析 #

EF Core会自动检测实体的所有公共属性并尝试映射到数据库列。你的TestBridgeEntity中同时存在抽象类的LeftId（保护内部属性）和子类的LeftSideEntityId（公共包装属性），EF会将两者都视为需要映射的属性，而扩展方法中把LeftId映射到left_side_id，EF默认又会将LeftSideEntityId映射到同名列，最终导致重复映射错误。

方法1：忽略包装属性的映射 #

修改扩展方法，添加对包装属性的忽略配置，同时通过表达式参数传入需要忽略的属性：

public static EntityTypeBuilder<T> ConfigureBridge<T,A,B,C,D>(this EntityTypeBuilder<T> builder, 
    string leftIdName, string rightIdName,
    Expression<Func<T, object>> leftNavigationWrapper,
    Expression<Func<T, object>> leftIdWrapper,
    Expression<Func<T, object>> rightNavigationWrapper,
    Expression<Func<T, object>> rightIdWrapper)
 where T : Bridge<A,B,C,D> where A: Entity<B> where C:Entity<D>
{
    // 忽略子类的包装属性
    builder.Ignore(leftNavigationWrapper);
    builder.Ignore(leftIdWrapper);
    builder.Ignore(rightNavigationWrapper);
    builder.Ignore(rightIdWrapper);

    builder.HasKey(x => new { x.LeftId, x.RightId } );

    builder.HasOne(x => x.Left)
           .WithMany()
           .HasForeignKey(x => x.LeftId);

    builder.HasOne(x => x.Right)
           .WithMany()
           .HasForeignKey(x => x.RightId);

    builder.Property(e => e.LeftId).HasColumnName(leftIdName);
    builder.Property(e => e.RightId).HasColumnName(rightIdName);

    return builder;
}

调用扩展方法时传入包装属性的表达式：

internal class MyTestBirdgeEntityConfiguration : IEntityTypeConfiguration<TestBridgeEntity>
{
    public void Configure(EntityTypeBuilder<TestBridgeEntity> builder)
    {
        builder.ToTable("test_bridge");
        builder.ConfigureBridge<TestBridgeEntity, LeftSideEntity, Guid, RightSideEntity, Guid>(
            "left_side_id", 
            "right_side_id",
            x => x.MyLeftSideEntity,
            x => x.LeftSideEntityId,
            x => x.MyRightSideEntity,
            x => x.RightSideEntityId);

        builder.Property(e => e.MyTestData ).IsRequired();
    }
}

方法2：调整抽象类与子类的属性设计 #

修改抽象类的属性为受保护，子类通过new关键字直接公开同名属性，避免包装属性导致的重复映射：

修改抽象类：

public abstract class Bridge<Q,R,S,T> where Q : Entity<R> where S : Entity<T>
{
    protected Q Left { get; set; }
    protected R LeftId { get; set; }
    protected S Right { get; set; }
    protected T RightId { get; set; }
}

子类实现：

public class TestBridgeEntity : Bridge<LeftSideEntity, Guid, RightSideEntity, Guid>
{
    public new LeftSideEntity Left 
    {
        get { return base.Left; } 
        set { base.Left = value; } 
    }
    
    public new Guid LeftId 
    {
        get { return base.LeftId; } 
        set { base.LeftId = value; } 
    }

    public new RightSideEntity Right 
    {
        get { return base.Right; } 
        set { base.Right = value; } 
    }

    public new Guid RightId 
    { 
        get { return base.RightId; } 
        set { base.RightId = value; } 
    }

    public double MyTestData { get; set; }
}

这种方式下，原扩展方法无需修改，EF只会映射子类的公共属性，不会出现重复映射问题。

内容的提问来源于stack exchange，提问作者WeLikeAich