抽象类与派生类条件映射问题导致EF6 SaveChanges报错

问题场景 #

使用EF6的TPH（每个层次结构一张表）继承映射模式，基类Collection为抽象类，子类NewsCollection继承自它，通过OfType<NewsCollection>查询实体并修改后，调用SaveChanges抛出MappingException，错误提示Condition member 'Collection.COL_TYPE' with a condition other than 'IsNull=False' is mapped。

错误原因 #

ColType字段同时被设置为复合主键的成员，又通过Map<NewsCollection>(co => co.Requires("ColType").HasValue(19999))作为子类的鉴别器条件，EF6不允许主键字段同时作为带固定值的鉴别器条件，导致映射视图生成冲突。

解决方案 #

1. 恢复子类的鉴别器属性重写 #

在NewsCollection类中重写ColType属性，固定返回鉴别器值19999，确保实体实例的鉴别器值正确：

public class NewsCollection : Collection
{
    public override int ColType => 19999;
}

2. 修改映射配置，使用TPH标准鉴别器配置 #

在CollectionDbMap中，替换原有的Map<NewsCollection>配置，改用HasDiscriminator来定义鉴别器字段和子类对应值：

public class CollectionDbMap : EntityTypeConfiguration<Collection>
{
    public CollectionDbMap()
    {
        ToTable("WWW_COLLECTION");
        HasKey(co => new { co.ColType, co.UserSn, co.ItemId });
        Property(co => co.ColType).HasColumnName("COL_TYPE");
        Property(co => co.UserSn).HasColumnName("USER_SN");
        Property(co => co.ItemId).HasColumnName("ITEM_ID");
        Property(co => co.CreatedBy).HasColumnName("CREATED_BY");
        Property(co => co.CreatedDate).HasColumnName("CREATED_DATE");
        Property(co => co.Title).HasColumnName("TITLE");
        
        // 配置TPH鉴别器
        HasDiscriminator<int>("COL_TYPE")
            .HasValue<NewsCollection>(19999);
    }
}

3. 验证查询与保存逻辑 #

修改后，db.Collections.OfType<NewsCollection>()会自动通过鉴别器值筛选出NewsCollection类型的实体，修改实体属性后调用SaveChanges时，EF6能正确识别实体类型并生成对应的更新语句，不会再出现映射冲突。

修改后的完整代码示例 #

using System;
using System.Data.Entity;
using System.Data.Entity.ModelConfiguration;
using System.Linq;

namespace ConsoleApp1
{
    internal class Program
    {
        static void Main(string[] args)
        {
            try
            {
                var db = new ToutiaoDbContext();
                var query = db.Collections.OfType<NewsCollection>();
                var list = query.ToList();
                var entity = list.FirstOrDefault();
                if(entity != null) // 增加空判断避免空引用
                {
                    entity.Title = "title123";
                    var res = db.SaveChanges();
                    Console.WriteLine(res);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }
    }

    public class ToutiaoDbContext : DbContext
    {
        public ToutiaoDbContext()
            : base("xxx")
        {
        }

        public DbSet<Collection> Collections { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Configurations.Add(new CollectionDbMap());
            base.OnModelCreating(modelBuilder);
        }
    }

    public class CollectionDbMap : EntityTypeConfiguration<Collection>
    {
        public CollectionDbMap()
        {
            ToTable("WWW_COLLECTION");
            HasKey(co => new { co.ColType, co.UserSn, co.ItemId });
            Property(co => co.ColType).HasColumnName("COL_TYPE");
            Property(co => co.UserSn).HasColumnName("USER_SN");
            Property(co => co.ItemId).HasColumnName("ITEM_ID");
            Property(co => co.CreatedBy).HasColumnName("CREATED_BY");
            Property(co => co.CreatedDate).HasColumnName("CREATED_DATE");
            Property(co => co.Title).HasColumnName("TITLE");
            
            // 配置TPH鉴别器
            HasDiscriminator<int>("COL_TYPE")
                .HasValue<NewsCollection>(19999);
        }
    }

    public abstract class Collection
    {
        public virtual int ColType { get; protected set; }
        public int UserSn { get; set; }
        public int ItemId { get; set; }
        public int CreatedBy { get; set; }
        public DateTime CreatedDate { get; set; }
        public string Title { get; set; }
    }

    public class NewsCollection : Collection
    {
        public override int ColType => 19999;
    }
}

内容的提问来源于stack exchange，提问作者huhmuren heriid