java - 帮助理解 hibernate 行为/错误

Question

我有一个 Product 类，它与自身具有一对多关联(复合模式)。下面分别是pojo、hbm、测试代码和日志/错误。有人可以解释这个错误是什么。为什么在设置父项的主键时会出现此错误？

产品展示

public class Product
{
    private Long id;

    private Set<Product> children = Collections.emptySet();

    public void addChild(final Product child)
    {
        if (children.isEmpty())
        {
            children = Sets.newHashSet();
        }
        child.setParent(this);
        children.add(child);
    }
}

hbm.xml

<class name="Product" table="PRODUCT">
        <set name="children" lazy="false" table="PRODUCT">
            <key>
                <column name="ID" />
            </key>
            <one-to-many class="Product" />
        </set>
</class>

测试

public void save()
    {
        // Level 1 - mortgage LOB
        Product mortgage = new Product();
        mortgage.setCode("Mortgage");

        Product ml = new Product();
        ml.setCode("Mortgage Loan");

        Product me = new Product();
        me.setCode("Home Equity LOC");

        //Level 2
        mortgage.addChild(ml);
        mortgage.addChild(me);

        hibernateTemplate.save(mortgage);
    }

日志和错误

DEBUG [org.hibernate.SQL] insert into PRODUCT (ID, CODE, NAME, STARTDATE, ENDDATE, ISDECISIONABLE, ISSELECTABLE) values (null, ?, ?, ?, ?, ?, ?)
DEBUG [org.hibernate.type.StringType] binding 'Mortgage' to parameter: 1
DEBUG [org.hibernate.type.StringType] binding null to parameter: 2
DEBUG [org.hibernate.type.TimestampType] binding null to parameter: 3
DEBUG [org.hibernate.type.TimestampType] binding null to parameter: 4
DEBUG [org.hibernate.type.BooleanType] binding 'false' to parameter: 5
DEBUG [org.hibernate.type.BooleanType] binding 'false' to parameter: 6
DEBUG [org.hibernate.SQL] call identity()
DEBUG [org.hibernate.SQL] update PRODUCT set ID=? where ID=?
DEBUG [org.hibernate.type.LongType] binding '1' to parameter: 1
ERROR [org.hibernate.event.def.AbstractFlushingEventListener] Could not synchronize database state with session
org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.Product
    at org.hibernate.engine.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:219)
    at org.hibernate.type.EntityType.getIdentifier(EntityType.java:397)
    at org.hibernate.type.ManyToOneType.nullSafeSet(ManyToOneType.java:78)

Answer 1

transient 对象是尚未保存的对象。正如映射的那样，您的关系不使用任何“传递持久性”，也就是说，您没有告诉 hibernate ，当您保存 parent 时，您想要保存 child 。我猜这个错误是因为你在父级(在 session 中)上有子级的实例(子级与父级具有相同的类无关紧要)，并且 hibernate 知道这一点，但它不知道如何处理它们。

您可以:

1) 在保存 parent 之前先保存 child
2) 将 'cascade' 属性添加到您的映射中，例如 cascade="save"或 cascade="all"

参见 http://docs.jboss.org/hibernate/core/3.3/reference/en/html/objectstate.html#objectstate-transitive