一对多关联的二级缓存

Question

I am using Hibernate 3.2.5.

I have a one to many relation between Department and Training tables. The second-level caching is enabled (Using EHCache) and the below entry is made in both the dept.cfg.xml and `training.hbm.xml files for caching the data.

<cache usage="read-only" />

Problem Description

For the first time, DB hit is done for getting both the Dept and Training records. The second time, the Department data is fetched from the cache but for getting the Training data, a DB hit is done again - WHY? I want this Training data also to be fetched from the cache rather than hitting the DB every time.

This is Dept.java file:

private int deptId;
private String deptName;
private Map trainingDetails;

I have mentioned the mapping in the dept.hbm.xml file as follows:

//Mappings between POJO and DB Table
<map name="trainingDetails" inverse="false" cascade="delete" lazy="false">
      <key column="DEPT_ID"></key>          
      <map-key formula="ID" type="integer"></map-key>
      <one-to-many class="com.model.Training"/>          
</map>

This is the code I tried:

SessionFactory sf = new Configuration().configure("trial.cfg.xml").buildSessionFactory();
    Session session = sf.openSession();

    Dept department = (Dept)session.load(Dept.class, 1);
    //Some business related operations
    session.flush();
    session.close();
            //Some operations
            Session session1 = sf.openSession();        
    Dept department1 = (Dept)session1.load(Dept.class, 1);
    //Here I can see in the console the query for fetching the 
    //training details for the department is getting executed again
    //but the Department details is getting fetched from the Cache - WHY?
    //I want the Training details also to be fetched from the cache.
    session1.flush();
    session1.close();

Kindly let me know what am I missing and how to resolve this.

Answer 1

If you tell Hibernate to cache Department entities in the second level cache, for each cached Department it will store the values for the deptId and deptName fields. However, it doesn't by default store the contents of the trainingDetails field. If a Department is read from the second level cache and the application needs to access the members field, Hibernate will go to the database to determine the current members of the collection.

If you want Hibernate to cache the contents of the members field, you need to tell it to do so by adding a cache element to the members declaration:

//Mappings between POJO and DB Table
<map name="trainingDetails" inverse="false" cascade="delete" lazy="false">
      <!-- Cache the ids of entities are members of this collection -->
      <cache usage="read-only" />
      <key column="DEPT_ID"></key>          
      <map-key formula="ID" type="integer"></map-key>
      <one-to-many class="com.model.Training"/>          
</map>