This document discusses mappings and queries in Hibernate, an object-relational mapping framework. It covers the core elements of a Hibernate application including Java objects, mapping files, and configuration files. It also discusses mapping collections and associations between objects, representing these relationships with join tables. Finally, it mentions querying objects using the HQL query language.

1. Mappings and Queries
with
Hibernate

2. Mappings and Queries
with
Hibernate

3. Introduction
*Hibernate is an object-relational mapping framework
*Maps persistence operations between object models to relational
databases
* Core elements in a Hibernate application are:
– Your Java objects
– The Hibernate object mapping files (Event.hbm.xml)
– The Hibernate configuration file (Hibernate.cfg.xml)
– The Hibernate configuration file (Hibernate.cfg.xml)
– Classes working with the Hibernate API (Session, Transaction)

4. Mapping
• Collection mapping
• Association mapping
• Component mapping

5. Collection mapping
• Collection properties must be declared as an interface type
(Set, not HashSet)
• May contain basic types custom types and references to other
Hibernate objects
• Collections are represented by a Collections are represented
by a collection table collection table in the Database
– Collection Key: foreign key of owning objects
– Collection element: object in the collection

6. Collection mapping
Java objects
Collection declared as
interface type
Hibernate mapping
Refers to Java
property
Foreign key to owner
Actual content of set
public class Person
{
private int id;
// other properties
private Set<String> emails;
// get and set methods
}
<<classname=”no.uio.inf5750.example.model.Person
table=”persons”>
<set name=”emails” table=” person _ emails”>
<key column=”person id”/>
<element column=”email” type=”string”/>
</set></class>

7. Association mapping
• Hibernate lets you easily specify all kinds of associations
between objects
–Unidirectional one -to –many
– Unidirectional many-to-many
– Bidirectional one-to-many
– Bidirectional
• Representing associations with join tables makes the
database schema cleaner
• Nullable foreign keys bad practise

8. The inverse property
• Bidirectional associations must be updated on both sides in the Java
code!
• Hibernate maps many-relationships with a join table
• Hibernate must ignore one side to avoid constraint violations!
• Must be many-side on one-to-many, doesn’t matter on many-to-many

9. Queries
• The Query interface
• The Hibernate Query Language
(HQL)
To Be Continued…….