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how to write queries in parent -child relationships using hibernate

Below is the mapping files for parnet and child tables;


Parent
*****

ID NAME
1  STEVEN
2  JOHNSON


Child
*****
ID FIRSTNAME      LASTNAME                                 PID
1  XXX                             AAA                                   1
2  YYYY                          LLL                         1
3  GGGG             JJJJ            2
4  JJJJ             BJB                                  1
5  HGHH             JUIU                          2

i have 2 qustions in this..

1.Using hibernate how to retrieve child records by passing corresponding parentid(pid);i want to know how to use hibernate in a good way?

2.how to write below queries in Hibernate??
select *  from parent p,child c where p.id=c.pid and c.pid=1



Parent.hbm.xml
**********
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.dao.Parent" table="PARENT" schema="SCOTT">
        <id name="pid" type="java.lang.Long">
            <column name="PID" precision="6" scale="0" />
            <generator class="increment" />
        </id>
        <property name="pname" type="java.lang.String">
            <column name="PNAME" length="50" />
        </property>
        <set name="childs" inverse="true">
            <key>
                <column name="PID" precision="6" scale="0" />
            </key>
            <one-to-many class="com.dao.Child" />
        </set>
    </class>
</hibernate-mapping>

Child.hbm.xml
************
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.dao.Child" table="CHILD" schema="SCOTT">
        <id name="cid" type="java.lang.Long">
            <column name="CID" precision="6" scale="0" />
            <generator class="increment" />
        </id>
        <many-to-one name="parent" class="com.dao.Parent" fetch="select">
            <column name="PID" precision="6" scale="0" />
        </many-to-one>
        <property name="firstname" type="java.lang.String">
            <column name="FIRSTNAME" length="12" />
        </property>
        <property name="lastname" type="java.lang.String">
            <column name="LASTNAME" length="12" />
        </property>
    </class>
</hibernate-mapping>
0
chaitu chaitu
Asked:
chaitu chaitu
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2 Solutions
 
mnrzCommented:
you may use Criteria:

Criteria c = session.createCriteria(Parent.class);
c.createAlias("childs","childs");
c.add(Expression.eq("parent.id",pid)); //pid in your case is 1
List result = c.list();
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chaitu chaituAuthor Commented:
c.createAlias("childs","childs");

what will it do?it will automatically join parent and child tables??

if i want outjoin on child table what should i do??
0
 
chaitu chaituAuthor Commented:
what is meaning of this --- ("childs","childs")
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mnrzCommented:
well
in your hbm you defined a one-to-many relationship

 <one-to-many class="com.dao.Child" />

so, hibernate will understand your child table

c.createAlias("childs","childs"); is just like "select * from Parent parent, Child childs"

and after that you can access all its fields by using the alias "childs", for example, "childs.firstName"

-----
for outer join, I am not sure but I think you should use this

c.createAlias("childs","childs",Criteria.LEFT_JOIN); or
c.createAlias("childs","childs",Criteria.FULL_JOIN);
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chaitu chaituAuthor Commented:
Criteria c  = getSession().createCriteria(Parent.class);
                   
              System.out.println("aaaaaaaaaaa11111");
            c.createAlias("childs","childs");
            System.out.println("aaaaaaaaaaa222222");
            c.add(Expression.eq("parent.pid",1));
            List results = c.list();

WHEN I execute i am getting below exception

aaaaaaaaaaa11111
aaaaaaaaaaa222222
Exception in thread "main" org.hibernate.QueryException: could not resolve property: parent of: com.dao.Parent
      at org.hibernate.persister.entity.AbstractPropertyMapping.throwPropertyException(AbstractPropertyMapping.java:43)
      at org.hibernate.persister.entity.AbstractPropertyMapping.toType(AbstractPropertyMapping.java:37)
      at org.hibernate.persister.entity.AbstractEntityPersister.getSubclassPropertyTableNumber(AbstractEntityPersister.java:1282)
      at org.hibernate.persister.entity.BasicEntityPropertyMapping.toColumns(BasicEntityPropertyMapping.java:31)
      at org.hibernate.persister.entity.AbstractEntityPersister.toColumns(AbstractEntityPersister.java:1257)
      at org.hibernate.loader.criteria.CriteriaQueryTranslator.getColumns(CriteriaQueryTranslator.java:433)
      at org.hibernate.loader.criteria.CriteriaQueryTranslator.getColumnsUsingProjection(CriteriaQueryTranslator.java:393)
      at org.hibernate.criterion.SimpleExpression.toSqlString(SimpleExpression.java:45)
      at org.hibernate.loader.criteria.CriteriaQueryTranslator.getWhereCondition(CriteriaQueryTranslator.java:333)
      at org.hibernate.loader.criteria.CriteriaJoinWalker.<init>(CriteriaJoinWalker.java:82)
      at org.hibernate.loader.criteria.CriteriaLoader.<init>(CriteriaLoader.java:67)
      



this is my Parent.java class
*****************************

                  public class Parent  implements java.io.Serializable {


    // Fields    

     private Long pid;
     private String pname;
     private Set childs = new HashSet(0);


    // Constructors

    /** default constructor */
    public Parent() {
    }

   
    /** full constructor */
    public Parent(String pname, Set childs) {
        this.pname = pname;
        this.childs = childs;
    }

   
    // Property accessors

    public Long getPid() {
        return this.pid;
    }
   
    public void setPid(Long pid) {
        this.pid = pid;
    }

    public String getPname() {
        return this.pname;
    }
   
    public void setPname(String pname) {
        this.pname = pname;
    }

    public Set getChilds() {
        return this.childs;
    }
   
    public void setChilds(Set childs) {
        this.childs = childs;
    }
   

}
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JaaxCommented:
1. Firstly you it is advisable to add  your "childs" as a SET  in your parent entity - something like this in the parent hbm file
....
<set name="children">
      <key column="PID"/>
      <one-to-many class="com.dao.Child"/>
</set>
....

Also define corresponding getters and setters in the Parent.java file

Having done so, the children can be retrieved as follows in java

Parent parent = (Parent)getSessionFactory().openSession().get(com.dao.Parent.class, parentId);
Hibernate.initialize(parent .getChildren());
if(parent .getChildren() != null){
    Iterator iter = parent .getChildren().iterator();
   while(iter.hasNext()){
       Hibernate.initialize(iter.next());
  }
}


2.You can use HQL for your queries :
String query = "select * from parent p
      left outer join p.children c
where  c.pid=1";

List list = HibernateTemplate.find(query); //Use an instance of HibernateTemplate
The list contains your results in the form of list of object arrays



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mnrzCommented:
sorry
please change this line

c.add(Expression.eq("parent.pid",1));

to

c.add(Expression.eq("pid",1));

please show your Child class
0
 
chaitu chaituAuthor Commented:
jaax,

i removed Hibernate.initialize in my code its working fine;what is the use of Hibernate.initialize ??

mnrz,

if i changed it to c.add(Expression.eq("pid",new Long(1)));  its working fine.
0
 
JaaxCommented:
Do you mean to say that with Hibernate.initialize , your code fails - i.e; throws some kind of Hibernate Exception ?

Hibernate.initialize() initializes the entities on demand and not beforehand, thus improving performance.
If you had specified lazy="false" in your hbm file, hibernate would load all the objects in the object graph and that would consume memory but accords the client, the luxury to use any of the objects at its whim. On the other hand, specifying lazy="true", postpones loading of the entity until actually it is needed (by specifying Hibernate.initialize()) and thus economizes on memory. But on the flip-side, the clinet should be careful to use only those objects in the graph that have been initialized.
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chaitu chaituAuthor Commented:

i observed two things by writing in both forms.


Parent parent = (Parent)getSessionFactory().openSession().get(com.dao.Parent.class, parentId);
Hibernate.initialize(parent .getChildren());
if(parent .getChildren() != null){
    Iterator iter = parent .getChildren().iterator();
   while(iter.hasNext()){
       Hibernate.initialize(iter.next());
  }
}


from writing above code actually it execting two queries like

select parent0_.PID as PID0_0_, parent0_.PNAME as PNAME0_0_ from PARENT parent0_ where parent0_.PID=?

select childs0_.PID as PID1_, childs0_.CID as CID1_, childs0_.CID as CID1_0_, childs0_.PID as PID1_0_, childs0_.FIRSTNAME as FIRSTNAME1_0_, childs0_.LASTNAME as LASTNAME1_0_ from CHILD childs0_ where childs0_.PID=?

*******************************************************************************

but the below code executes only one query by joining two tables like....thus improving performace.


select this_.PID as PID0_1_, this_.PNAME as PNAME0_1_, childs1_.CID as CID1_0_, childs1_.PID as PID1_0_, childs1_.FIRSTNAME as FIRSTNAME1_0_, childs1_.LASTNAME as LASTNAME1_0_ from PARENT this_ inner join CHILD childs1_ on this_.PID=childs1_.PID where this_.PID=?

Criteria c  = getSession().createCriteria(Parent.class);
            c.createAlias("childs","childs");
            c.add(Expression.eq("pid",new Long(1))); //pid in your case is 1
            List results = c.list();
           
            for (Iterator iter = results.iterator(); iter.hasNext();) {
                        Child element = (Child) iter.next();
                          System.out.println("ParentDAO.findByExample()"+element.getFirstname());
                  }

Exception in thread "main" java.lang.ClassCastException: com.dao.Parent
      

but my problem is how to get only child information ;if i type cast with Parent object its working fine...but i want child objects ..


0
 
mnrzCommented:
so you can create your criteria with Child, check this:

List result = session.createCriteria(Child.class).
createAlias("parent","parent").
add(Expression.eq("parent.pid",new Long(1))).
list();

this will return a list of type "Child"

0
 
JaaxCommented:
After you have done this :

Parent parent = (Parent)getSessionFactory().openSession().get(com.dao.Parent.class, parentId);
Hibernate.initialize(parent .getChildren());
if(parent .getChildren() != null){
    Iterator iter = parent .getChildren().iterator();
   while(iter.hasNext()){
       Hibernate.initialize(iter.next());
  }
   
   Iterator iter = parent .getChildren().iterator();
  while(iter.hasNext()){
      Child child = (Child)iter.next();
     //Do what you like with child entity
    }

   
 

}


}

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chaitu chaituAuthor Commented:
final question for both of you which approach is better ;my personnel view is mnrz is said is better one because its executing only query gives us better performance what do you say Jaax..
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mnrzCommented:
In my opinion, the performance in both of them is alike, but with Criteria you have many options and features you may need in the future
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JaaxCommented:
The choice should be on the requirement. Load is useful when you are querying via PRIMARY KEY and when you are damn sure that it will result in only one record.
Criteria is mainly used for SEARCHING and as mnrz has maintained, additional search attributes can be added or removed as you like. Criteria is the Object oriented way of dealing with DB search. However, Criteria is restricted in certain aspects - like you cannot specify SQL functions and limited flexibility with joins.

To summarize:
1.If you want to load only one record based on primary key go for load or get
2.If you want to search and are expecting multiple outputs, then opt for Criteria or HQL
The choice between Criteria and HQL should be based on flexibility in terms of using advanced SQL features (HQL) and using Object oriented way of searching
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