java Hibernate查询问题

Question

我正在使用Hibernate。我的数据库如下

类别有许多属性

类别 包含

private Set <Attribute> AllAttributes= new HashSet  <Attribute>();

类属性

我如何检索所有类别及其属性，因为我正在尝试“从类别”但它不起作用

类别映射文件：

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Dec 16, 2010 8:37:02 AM by Hibernate Tools 3.4.0.Beta1 -->
<hibernate-mapping>
    <class name="com.BiddingSystem.Models.Category" table="CATEGORY">
        <id name="CategoryId" type="long">
            <column name="CATEGORYID" />
            <generator class="native" />
        </id>
        <property name="CategoryName" type="java.lang.String">
            <column name="CATEGORYNAME" />
        </property>

        <many-to-one name="ParentCategory" class="com.BiddingSystem.Models.Category">
            <column name="PARENT_CATEGORY_ID" />
        </many-to-one>

        <set name="SubCategory" lazy="false" cascade="all-delete-orphan" inverse="true">
            <key>
                <column name="PARENT_CATEGORY_ID" />
            </key>
            <one-to-many class="com.BiddingSystem.Models.Category" />
        </set>

        <set name="AllAttributes" table="ATTRIBUTE" inverse="false" lazy="true"  cascade="all">
            <key>
                <column name="CATEGORYID" />
            </key>
            <one-to-many class="com.BiddingSystem.Models.Attribute" />
        </set>

    </class>
</hibernate-mapping>

属性映射文件：

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Dec 16, 2010 5:25:09 AM by Hibernate Tools 3.4.0.Beta1 -->
<hibernate-mapping>
    <class name="com.BiddingSystem.Models.Attribute" table="ATTRIBUTE">
        <id name="AttributeId" type="long">
            <column name="ATTRIBUTEID" />
            <generator class="native" />
        </id>
        <property name="AttributeName" type="java.lang.String">
            <column name="ATTRIBUTENAME" />
        </property>
        <set name="Options" table="ATTRIBUTEOPTION" inverse="false"  cascade="all">
            <key>
                <column name="ATTRIBUTEID" />
            </key>
            <one-to-many class="com.BiddingSystem.Models.AttributeOption" />
        </set>
    </class>
</hibernate-mapping>

Answer 1

您已将关联映射到lazy="true"。这告诉hibernate，默认情况下，类别的属性只应在实际访问时从数据库中加载，而lazy="false"则指示hibernate在加载类别时加载属性。但是，映射文件中的指令会影响所有查询

如果您只想将其用于特定查询，请签出 http://docs.jboss.org/hibernate/core/3.5/reference/en/html/queryhql.html#queryhql-joins：

A "fetch" join allows associations or collections of values to be initialized along with their parent objects using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations of the mapping file for associations and collections. See Section 20.1, “Fetching strategies” for more information.

from Cat as cat
    inner join fetch cat.mate
    left join fetch cat.kittens

A fetch join does not usually need to assign an alias, because the associated objects should not be used in the where clause (or any other clause). The associated objects are also not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason you might need an alias is if you are recursively join fetching a further collection:

from Cat as cat
    inner join fetch cat.mate
    left join fetch cat.kittens child
    left join fetch child.kittens

The fetch construct cannot be used in queries called using iterate() (though scroll() can be used). Fetch should be used together with setMaxResults() or setFirstResult(), as these operations are based on the result rows which usually contain duplicates for eager collection fetching, hence, the number of rows is not what you would expect. Fetch should also not be used together with impromptu with condition. It is possible to create a cartesian product by join fetching more than one collection in a query, so take care in this case. Join fetching multiple collection roles can produce unexpected results for bag mappings, so user discretion is advised when formulating queries in this case. Finally, note that full join fetch and right join fetch are not meaningful.