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Tags/Keywords : Hibernate Mapping Question, Hibernate, Mapping, Question,one-to-one,mapping question
Author : Amit
Date (Year/Month/Date): 2010-04-03
Hibernate one to one type mapping question with
my answer, please do comment on this please.

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Hibernate mapping interview question :

Recently someone asked me a very basic question, but actually
I was little bit confused to reply answer to this
particular question.

This questions is targeted towards the mapping of POJO
bean with the corresponding table from persistence
in an one to one style of Hibernate mapping.

The question is as follows:

  Can there be Hibernate mapping possible between two
  POJOs using two different tables in persistence,
  whereby these tables are having primary keys, but
  no associated foreign key in either table?

  If I try to elaborate taking a simple example, there
  are two POJOs A and B, are to be associated with
  each other in form of Mapping, whereas table for A
  and B are for example TA and TB respectively.And
  TA is having a primary key as sample_id_a and TB has
  primary key as sample_id_b. but these tables are not
  related with each other in any kind of relational
  constraint, such as primarykey-foreignkey constraint.

  Is this possible??

Well I shall try to show that it is possible to have
one to one mapping (have not tried any other type of
mapping as yet) between these two POJOs in form of
association.

Looking at the Hibernate one to one specific mapping
from one of the example already shown in this site:


<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="example.iqtf.domain">
<class name="Book" table="book">
<id name="bookId" access="property" column="book_id"/>
<property name="bookTitle" access="property"
                              column="book_name"/>
<one-to-one name="category"
               class="Category" cascade="all"/>
</class>

<class name="Category" table="category" >
<id name="categoryId" access="property"
                         column="category_id"/>
<property name="categoryTitle" access="property"
                         column="category_title"/>
</class>
</hibernate-mapping>



In this HBM file, we have two POJOs are Book and Category,
associated with each other in a one to one type of mapping.

So Book has a instance variable of type as Category.
Book class is configured to be persisted in table "book",
with book_id as primary key, and Category class persisted
in table category, with the category_id as primary key
constraint defined.

one-to-one tag in Book class tag definition in this HBM
file has the name as category, this means the variable
name is category with type as Category class in the Book
class. So I don't see any mention of foreign key in this
type of mapping. But if the constraint attribute in the
one-to-one mapping is set as "true", then it is required
to have a foreign key constraint defined in the category
table for the book_id primary key in book table.

Hope this helps in finding out one out of many answers.

If you have anything to add to this, you are most welcome.

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