定义含自引用实体的EF Core 3.1模型时迁移异常问题

问题场景 #

被迫使用EF Core 3.1版本，需要定义两个独立数据表，每个表包含指向同表内其他实体的自引用外键（用于实现实体复制逻辑，让每个实体知晓其祖先），希望通过抽象基类统一配置实体的主键和自引用关系，避免重复代码。

初始实现代码 #

实体类代码 #

public abstract class EntityBase
{
    public int Id { get; set; }
    public string Name { get; set; }

    public int? AncestorId { get; set; }
    public virtual EntityBase Ancestor { get; set; }

    internal static void OnModelCreating<T>(ModelBuilder modelBuilder) where T : EntityBase
    {
        var entity = modelBuilder.Entity<T>();
        entity.HasKey(e => e.Id);

        entity.HasOne(e => e.Ancestor)
            .WithMany()
            .HasForeignKey(e => e.AncestorId)
            .OnDelete(DeleteBehavior.Restrict);
    }
}

public class EntityA:EntityBase
{
    public string PropertyA { get; set; }
}

public class EntityB:EntityBase
{
    public string PropertyB { get; set; }
}

DbContext代码 #

public class TestDbContext : DbContext
{
    private readonly string connectionString = // .... 省略;

    public DbSet<EntityA> EntitiesA { get; set; }
    public DbSet<EntityB> EntitiesB { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        base.OnConfiguring(optionsBuilder); 
        optionsBuilder.UseSqlServer(connectionString);
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        EntityBase.OnModelCreating<EntityA>(modelBuilder);
        EntityBase.OnModelCreating<EntityB>(modelBuilder);

        // 注释掉的单独配置代码
        //modelBuilder.Entity<EntityA>(entity =>
        //{
        //    entity.HasKey(e => e.Id);

        //    entity.HasOne(e => e.Ancestor)
        //        .WithMany()
        //        .HasForeignKey(e => e.AncestorId)
        //        .OnDelete(DeleteBehavior.Restrict);
        //});

        //modelBuilder.Entity<EntityB>(entity =>
        //{
        //    entity.HasKey(e => e.Id);

        //    entity.HasOne(e => e.Ancestor)
        //        .WithMany()
        //        .HasForeignKey(e => e.AncestorId)
        //        .OnDelete(DeleteBehavior.Restrict);
        //});
    }
}

迁移报错信息 #

执行首次迁移时触发以下错误：

Build started...
Build succeeded.
System.InvalidOperationException: A key cannot be configured on 'EntityA' because it is a derived type. The key must be configured on the root type 'EntityBase'. If you did not intend for 'EntityBase' to be included in the model, ensure that it is not included in a DbSet property on your context, referenced in a configuration call to ModelBuilder, or referenced from a navigation property on a type that is included in the model.
......

问题根源在于EntityBase中的导航属性Ancestor类型为EntityBase，EF Core会将其识别为继承层次结构的根类，进而要求主键必须配置在根类上，但实际需求是让EntityA和EntityB各自成为独立的表，而非共享继承表结构。尝试单独在DbContext中配置每个实体也无法解决问题。

可行解决方案：泛型基类 #

通过将抽象基类改为泛型类型，让每个派生类的自引用导航属性指向自身类型，避免EF Core将其识别为继承层次结构。

修改后的实体类代码 #

public abstract class EntityBase<T> where T : EntityBase<T>
{
    public int Id { get; set; }
    public string Name { get; set; }

    public int? AncestorId { get; set; }
    public T Ancestor { get; set; }

    internal static void OnModelCreating(ModelBuilder modelBuilder)
    {
        var entity = modelBuilder.Entity<T>();
        entity.HasKey(e => e.Id);

        entity.HasOne(e => e.Ancestor)
            .WithMany()
            .HasForeignKey(e => e.AncestorId)
            .OnDelete(DeleteBehavior.Restrict);
    }
}

public class EntityA:EntityBase<EntityA>
{
    public string PropertyA { get; set; }
}

public class EntityB:EntityBase<EntityB>
{
    public string PropertyB { get; set; }
}

修改后的DbContext代码 #

public class TestDbContext : DbContext
{
    private readonly string connectionString = // .... 省略;

    public DbSet<EntityA> EntitiesA { get; set; }
    public DbSet<EntityB> EntitiesB { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        base.OnConfiguring(optionsBuilder); 
        optionsBuilder.UseSqlServer(connectionString);
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        EntityBase<EntityA>.OnModelCreating(modelBuilder);
        EntityBase<EntityB>.OnModelCreating(modelBuilder);
    }
}

修改后，EF Core会将EntityA和EntityB视为完全独立的实体类型，各自生成独立的数据表，同时每个表都正确配置了自引用的外键关系，满足需求。

内容的提问来源于stack exchange，提问作者Paolo Martinello