cphbuscossem3/week4-orm-jpa

This repo contains all the material for 3rd semester 4th week about Object Relational Mapping (ORM) using JPA with our java applications

#ORM (Object Relational Mapping) with JPA (Java Persistence API) ##Step by step:
Purpose: Simple walk through of JPA Steps:

1. Create the project.
2. Create a database and connect (for this demo I use mysql database: jpa1)
3. Add the MYSQL JDBC Driver to the project libraries
4. Create a new Persistance Unit (right click project and choose new..-> Persistence ...)

• choose EclipseLink JPA 2.1 (Should be the default choice for the Persistence Provider)

5. Create new file -> Entity Class.

• Name it Person. and give it fields (ID, age, name)
• Run this class to create a db table from the entities:

public class Creater {
    public static void main(String[] args) {
        Persistence.generateSchema("JPADemo1PU", null); //Change PU name
    }
}

• Then create a class with an EntityManager based on the Persistence Unit See how running the persist() method creates a new table in the database called person with ID, Age and Name.
• To get the created SQL from EclipseLink insert the following 2 property lines in to the persistence.xml. :

Resources:
Tutorialspoint on JPA Simple JPA
More detailed JPA - this one provides detailed information

Day 1: Introduction to ORM

Tutorial for JPA

• Java Persistence API is a source to store business entities (java objects) as relational entities (db tables)

• Challenges:

  • Table structure in db vs graph structure in objects (references containing references ...)
  • Inheritance: Objects with inheritance hierarchy to be mirrored in tables

• JPA providers:

  • Hybernate
  • EclipseLink
  • Toplink

• JPA architecture: javax.Persistence !alt text

• Annotations

  • See list of annotations

• Persistence.xml

  • registers the database and specify the entity class

• Getting started: set up the first project.

###Exercise 1 - Basic JPA Find the exercise here

##Day 2 - More JPA

• JPA mapping directions

Cardinality	Direction	Example
One-to-one	Unidirectional	person and car
One-to-one	Bidirectional	person and spouse
One-to-many	Unidirectional	workplace and work station
Many-to-one/one-to-many	Bidirectional	student and teacher
Many-to-one	Unidirectional	student and school
Many-to-many	Unidirectional	person and phone
Many-to-many	Bidirectional	employee and company car

• In relational database it is always a unidirectional relationship based on foreign keys
• In OO Programming Objects can have reference to each other

//In the Student class
private Teacher teacher;
//In the Teacher class
private List<Student> students;

###Exercise 2 - Relationships Find the exercise here

##Day 3 - JPA and Inheritance

• The 3 strategies for mapping inheritance in database tables

###Exercise 3 - Inheritance Find the exercise here

##Day 4 - JPQL

• Java Persitence Query Language
• Query the managed objects using the Entity Manager
• Like SQL
• Query Objects and fields instead of tables and collumns

###Exercise 4 - JPQL Find the exercise here