集合例子

深碍√TFBOYSˉ_ 2022-01-13 23:55 295阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

package eg;
    import java.util.Set;
    
    public class Parent {
        private long id;
        private Set children;
    
        public long getId() { return id; }
        private void setId(long id) { this.id=id; }
    
        private Set getChildren() { return children; }
        private void setChildren(Set children) { this.children=children; }
    
        ....
        ....
    }

这个类有一个`Child`的实例集合。如果每一个子实例至多有一个父实例, 那么最自然的映射是一个one-to-many的关联关系：

<hibernate-mapping>
    
        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children">
                <key column="parent_id"/>
                <one-to-many class="Child"/>
            </set>
        </class>
    
        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
        </class>
    
    </hibernate-mapping>

在以下的表定义中反应了这个映射关系：

create table parent ( id bigint not null primary key )
    create table child ( id bigint not null primary key, name varchar(255), parent_id bigint )
    alter table child add constraint childfk0 (parent_id) references parent

如果父亲是*必须*的, 那么就可以使用双向one-to-many的关联了：

<hibernate-mapping>
    
        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children" inverse="true">
                <key column="parent_id"/>
                <one-to-many class="Child"/>
            </set>
        </class>
    
        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
            <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/>
        </class>
    
    </hibernate-mapping>

请注意`NOT NULL`的约束:

create table parent ( id bigint not null primary key )
    create table child ( id bigint not null
                         primary key,
                         name varchar(255),
                         parent_id bigint not null )
    alter table child add constraint childfk0 (parent_id) references parent

另外，如果你绝对坚持这个关联应该是单向的，你可以对`<key>`映射声明`NOT NULL`约束：

<hibernate-mapping>
    
        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children">
                <key column="parent_id" not-null="true"/>
                <one-to-many class="Child"/>
            </set>
        </class>
    
        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
        </class>
    
    </hibernate-mapping>

另外一方面,如果一个子实例可能有多个父实例, 那么就应该使用many-to-many关联：

<hibernate-mapping>
    
        <class name="Parent">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <set name="children" table="childset">
                <key column="parent_id"/>
                <many-to-many class="Child" column="child_id"/>
            </set>
        </class>
    
        <class name="Child">
            <id name="id">
                <generator class="sequence"/>
            </id>
            <property name="name"/>
        </class>
    
    </hibernate-mapping>

表定义：

create table parent ( id bigint not null primary key )
    create table child ( id bigint not null primary key, name varchar(255) )
    create table childset ( parent_id bigint not null,
                            child_id bigint not null,
                            primary key ( parent_id, child_id ) )
    alter table childset add constraint childsetfk0 (parent_id) references parent
    alter table childset add constraint childsetfk1 (child_id) references child

转载于:https://my.oschina.net/u/2552902/blog/543909

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220113/5cc1dbb562894d28a518015f7e39119f.png