Hibernate之deleted object would be re-saved by cascade异常

在Hibernate中，删除存在关联关系的一个对象时，会出现 org.hibernate.ObjectDeletedException: deleted object would be re-saved by cascade (remove deleted object from associations)这个异常

如下：
持久化类：

import java.util.HashSet;
import java.util.Set;

import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;
import org.apache.commons.lang.builder.ToStringBuilder;

/**
 * 产品类别对象
 * @author crazycy
 *
 */
public class ProductCategory extends BaseObject {

    // Fields    

    private long id;

    private String name;

    private String description;

    private ProductCategory parentCategory;

    private Set childrenCategories = new HashSet();

    // Constructors

    /** default constructor */
    public ProductCategory() {
    }

    /** minimal constructor */
    public ProductCategory(String name) {
        this.name = name;
    }
    
    public ProductCategory(String name, String description) {
        this.name = name;
        this.description = description;
    }

    /** full constructor */
    public ProductCategory(String name, String description,
            ProductCategory parentCategory) {
        this.name = name;
        this.description = description;
        this.parentCategory = parentCategory;
    }

    /** full constructor */
    public ProductCategory(String name, String description,
            Set childrenCategories) {
        this.name = name;
        this.description = description;
        this.childrenCategories = childrenCategories;
    }

    // Property accessors

    public long getId() {
        return this.id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return this.description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public ProductCategory getParentCategory() {
        return this.parentCategory;
    }

    public void setParentCategory(ProductCategory parentCategory) {
        this.parentCategory = parentCategory;
    }
    
    /**
     * 由主来调：是主添加
     * @param productCategory
     */
    public void addCategory(ProductCategory productCategory) {
        productCategory.setParentCategory(this);
        childrenCategories.add(productCategory);
    }
    
    /**
     * 由主来调；是从主删除
     * @param productCategory
     */
    public void removeCategory(ProductCategory productCategory) {
        childrenCategories.remove(productCategory);
        productCategory.setParentCategory(null);
    }
    
    public Set getChildrenCategories() {
        return childrenCategories;
    }

    public void setChildrenCategories(Set childrenCategories) {
        this.childrenCategories = childrenCategories;
    }

    /**
     * @see java.lang.Object#equals(Object)
     */
    public boolean equals(Object object) {
        if (!(object instanceof ProductCategory)) {
            return false;
        }
        ProductCategory rhs = (ProductCategory) object;
        return new EqualsBuilder().append(this.description, rhs.description)
                .append(this.name, rhs.name).append(this.id, rhs.id).isEquals();
    }

    /**
     * @see java.lang.Object#hashCode()
     */
    public int hashCode() {
        return new HashCodeBuilder(1009592109, -669108101).append(
                this.description).append(this.name).append(this.id)
                .toHashCode();
    }

    /**
     * @see java.lang.Object#toString()
     */
    public String toString() {
        return new ToStringBuilder(this).append("name", this.name).append(
                "description", this.description).append("id", this.id)
                .toString();
    }

}

映射文件

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="">
    <class name="ProductCategory" table="productcategory">
        <id name="id" type="long">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="string">
            <column name="name" length="50" not-null="true" />
        </property>
        <property name="description" type="string">
            <column name="description" length="150" />
        </property>
        <set name="childrenCategories" cascade="save-update" inverse="true">
            <key column="parent"/>
            <one-to-many class="ProductCategory"/>
        </set>
        <many-to-one name="parentCategory" column="parent" 
            class="ProductCategory" 
            cascade="save-update"
         >
        </many-to-one>
    </class>
</hibernate-mapping>

测试代码：

category2.getChildrenCategories().remove(category5);
        category5.setParentCategory(null);
        dao.removeProductCategory(category5.getId());

解决方案如下：
方法1 删除Set方的cascade
方法2 解决关联关系后，再删除 :

category2.getChildrenCategories().remove(category5);
        category5.setParentCategory(null);
        dao.removeProductCategory(category5.getId());

方法3 在many-to-one方增加cascade 但值不能是none

如果以上三个方案都失败（哼哼~ 我用了5个小时才找出来的）
检查一下hashCode equals是否使用了id作为唯一标示的选项了；我用uuid.hex时是没有问题的；
但是用了native，就不行了，怎么办？删除啊！

也就是问题出现在本文给出的持久化类的hashCode equals方法身上