Hibernate mapping multiple classes to one table using hbm.xml

Question

I am fairly new to Hibernate and need some help with hibernate-mapping.

I have 4 different classes which I want to map into one table, of which the primary key consists of attributes from 2 different classes. At the same time, I want to map only selected attributes from each class into a local database. I wish to avoid JPA annotations and define the mapping style in a hbm.xml file instead. How do I do that?

Take the following example:

public class Tenant implements Serializable {
    private final static long serialVersionUID = 1L;
    protected List<Rack> rack;
    protected String type;
    //getters setters
}

public class Rack implements Serializable {
    private final static long serialVersionUID = 1L;        
    protected List<Circuit> circuit;
    protected String rackLabel;
    protected Boolean excludes;
    //getters setters
}

public class Circuit implements Serializable {
    private final static long serialVersionUID = 1L;
    protected List<CircuitReadings> circuitReadings;
    protected String circuitNo;
    protected Boolean excludes;
    //getters setters
}

public class CircuitReadings
    implements Serializable {
    private final static long serialVersionUID = 1L;
    protected String date;
    protected String kva;
    protected String current;
    protected String kwh;
    //getters setters
}

And the eventual table should consist of the following:

    type | rackLabel | circuitNo | date | kva | current | energy

"circuitNo" and "date" above should form the composite primary keys.

Can someone show me an example of how I should map this? Thanks!

Why add all four classes in a single table? From the way they look, all classes have a many to one relationship between them. Adding all of them in a single table would yield a very de-normalized table. This is usually a bad thing in relational databases. — Ioan Alexandru Cucu
– Ioan Alexandru Cucu, Commented Jan 5, 2012 at 7:07
This is because I don't want to store all attributes to the database, and want only selected ones. the attributes I want are highlighted in the table description above. My solution was to create a separate method that loops through the "Tenant" object and its sub classes, and consolidate only values I want stored into the database into a separate class e.g. ClassA. A hbm file is then created for ClassA. I know it's not ideal to do this, hence, i'm looking for a more optimal solution. — louis xie
– louis xie, Commented Jan 13, 2012 at 7:25

Abhishek Kumar · Accepted Answer · 2013-03-04 05:49:22Z

Hibernate provide a way to map subclass using discriminator keyword.

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        <join table="CASH_PAYMENT">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        <join table="CHEQUE_PAYMENT" fetch="select">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
</class>

Aravind A · Accepted Answer · 2012-01-05 07:45:58Z

0

There's nothing stopping you from doing that . Create 4 HBMs pointing to the same table but different Pojos . Though it can be done , as @Ioan Alexandru Cucu , it is not recommended .

   <!-- HBM1-->

<class name="com.myProject.Rack"
    table="My_Table">

   <!-- HBM2-->

   <class name="com.myProject.Rack"
    table="My_Table">

answered Jan 5, 2012 at 7:45

Aravind A

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1 Comment

louis xie Over a year ago

I'm trying to understand how this would work. Referring back to my example,, how do I specify in the hbm that I want to repeat the same "circuitNo" for each "date", "kva", "current", and "energy" in the "CircuitReadings" list?

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