在 NHibernate 中将空字符串映射到 NULL [英] Mapping empty strings to NULL in NHibernate

问题描述

我有一个带有递归表的 SQL Server 数据库:

I have a SQL Server DB with a recursive table:

MyTable:
 ID : string PrimaryKey
 Parent: string references MyTable - NOTNULL !!

并使用 Fluent NHibernate 映射到

and map with Fluent NHibernate to

class MyTable
{
  public virtual string ID {get; set;}
  public virtual MyTable Parent {get; set;}
}

我的问题是，如果 Parent 列在数据库中为 ""(空字符串)，则 Parent 在我的 C# 应用程序中应该为空，反之亦然.不幸的是，我无法更改列类型以接受 NULL！

My problem is that Parent should be null in my C# app if the column Parent is "" (empty string) in the database and vice versa. Unfortunately I can't change the column type to accept NULL!

我尝试使用 IEmptyInterceptor，但无法正常工作.

I tried to use IEmptyInterceptor but I don't get it working.

提前致谢，叉

推荐答案

您需要有一个 IUserType 用于主键列，它执行特殊的 NULL 值处理.

You need to have an IUserType for the primary key column, which does the special NULL value handling.

public MyTableMap()
{
    Id(x => x.EntryNo)
        // Since the PK is a string, it must be assigned by the application.
        .GeneratedBy.Assigned()
        .SetAttribute("type", typeof(SpecialNullValueStringType).AssemblyQualifiedName);

    References(x => x.Parent);
}

public class SpecialNullValueStringType : IUserType
{
    #region IUserType Members
    public bool IsMutable
    {
        get { return false; }
    }

    public Type ReturnedType
    {
        get { return typeof(string); }
    }

    public SqlType[] SqlTypes
    {
        get { return new[] { NHibernateUtil.String.SqlType }; }
    }

    public object NullSafeGet(IDataReader rs, string[] names, object owner)
    {
        var obj = NHibernateUtil.String.NullSafeGet(rs, names[0]);

        if (obj == null)
        {
            return null;
        }

        var value = (string) obj;
        if (String.IsNullOrEmpty(value))
        {
            return null;
        }

        return value;
    }

    public void NullSafeSet(IDbCommand cmd, object value, int index)
    {
        if (value == null)
        {
            ((IDataParameter) cmd.Parameters[index]).Value = String.Empty;
        }
        else
        {
            ((IDataParameter) cmd.Parameters[index]).Value = value;
        }
    }

    public object DeepCopy(object value)
    {
        return value;
    }

    public object Replace(object original, object target, object owner)
    {
        return original;
    }

    public object Assemble(object cached, object owner)
    {
        return cached;
    }

    public object Disassemble(object value)
    {
        return value;
    }

    public new bool Equals(object x, object y)
    {
        if (ReferenceEquals(x, y))
        {
            return true;
        }

        if (x == null || y == null)
        {
            return false;
        }

        return x.Equals(y);
    }

    public int GetHashCode(object x)
    {
        return x == null ? typeof(string).GetHashCode() + 473 : x.GetHashCode();
    }
    #endregion
}

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