使用相同的数据透视表使用子类休眠多对多

问题描述

具有此模型架构:

Person
    |__ Student
            |__ SchoolBoy
            |__ CollegeStudent

我正在使用Hibernate 3.6，并且使用一个鉴别符列对所有类使用 tperson 表.我的映射是这样完成的:

I'm using Hibernate 3.6 and I use the tperson table for all the classes, using a discriminator column. My mapping is done like that:

<class name="Person" table="tperson" discriminator-value="PERSON">
    <id name="Id" column="id" type="integer">
        <generator class="increment" />
    </id>
    <discriminator column="person_type" />
    <subclass name="Student" discriminator-value="STUDENT">
        <key column="id_person" />
        <subclass name="SchoolBoy" discriminator-value="SCHOOL_BOY">
            <join table="tstudent">
                <key column="id_person" />
            </join>
        </subclass>
        <subclass name="CollegeStudent" discriminator-value="COLLEGE_STUDENT">
            <join table="tstudent">
                <key column="id_person" />
            </join>
        </subclass>
    </subclass>
</class>

现在，我想介绍 Course 实体，实现课程与学生之间的关系.当然，这是一个多对多的关系.假设我使用一个名为tstudent_course的数据透视表，其中包含两种类型的学生: SchoolBoy 和 CollegeStudent .该表包含对该人本身及其所学习课程的引用.

Now I want to introduce the Course entity, implementing a relation between courses and students. Of course, this is a many-to-many relation. Let's suppose I use a pivot table named tstudent_course, which contains students of both types SchoolBoy and CollegeStudent. This table contains a reference to the person itself and the course he's studying.

现在，当我加载 Course 实体时，我想在大学生和学校学生之间有所不同.我是那样做的:

Now I want to differ between college and school students when I load the Course entity. I do it like that:

<set name="CollegeStudents" table="tstudent_course"
            inverse="true">
    <key>
        <column name="id_course" not-null="true" />
    </key>
    <many-to-many entity-name="CollegeStudent">
        <column name="id_person" not-null="true" />
    </many-to-many>
</set>

<set name="SchoolStudents" table="tstudent_course"
            inverse="true">
    <key>
        <column name="id_course" not-null="true" />
    </key>
    <many-to-many entity-name="SchoolBoy">
        <column name="id_person" not-null="true" />
    </many-to-many>
</set>

但是，作为数据透视表的表是包含对每种类型学生的引用的表，它尝试加载我的收藏集中的每个学生，并且我收到下一个异常:

However, being the pivot table a table which contains references to every type of students, it tries to load every single student in my collections and I receive the next Exception:

Object with id: 2 was not of the specified subclass:
   CollegeStudent (loaded object was of wrong class class SchoolBoy)

Hibernate似乎在不评估我所拥有的具体学生类型的情况下进行参加，并试图在我收集的大学生中注入一个SchoolBoy.

It seems Hibernate is doing the join whithout evaluating the concrete type of student I have and tries to inject an SchoolBoy in my collection of College Students.

我该怎么做才能避免这种情况?是否可以在数据透视表中建立一种歧视?还是我必须为每种子类创建一个特定的数据透视表?

What can I do to avoid that? Is it possible to stablish a kind of discrimination in the pivot table? Or do I have to create an specific pivot table for each kind of subclass?

推荐答案

在您的集合中，您可以添加过滤器:

In your set you can add a filter:

<set name="CollegeStudents" table="tstudent_course"
            inverse="true">
    <key>
        <column name="id_course" not-null="true" />
    </key>
    <many-to-many entity-name="CollegeStudent" where="person_type='COLLEGE_STUDENT'">
        <column name="id_person" not-null="true" />
    </many-to-many>
</set>

恕我直言，如果没有该过滤器(只有一组学生)，映射会更好.

IMHO the mapping would be better without that filter (just a set of all students).

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