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3. Java Persistence API. 4. Entity Relationships

1. 3. Java Persistence API

4. Entity Relationships

2. Basics

• A foreign key can be one or more
columns that reference a unique key,
usually the primary key, in another table
• A foreign key and the primary parent key it
references must have the same number
and type of fields
• Foreign keys represents relationships
from a column or columns in one table to a
column or columns in another table
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3. Multiplicity in Entity Relationships

• Multiplicities are of the following types:
– Many-to-one: Multiple instances of an entity
can be related to a single instance of the
other entity
– One-to-one: Each entity instance is related to
a single instance of another entity
– One-to-many: An entity instance can be
related to multiple instances of the other
entities
– Many-to-many: The entity instances can be
related to multiple instances of each other.
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4. CM Database Schema

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5. Many-to-One Mappings

• A many-to-one mapping is defined by
annotating the attribute in the source entity
(the attribute that refers to the target entity)
with the @ManyToOne annotation
• A @JoinColumn(name=Fkname) annotation
discribes a foreing key of a relationship
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6. Exercise: Read a Payment

• Create a project for read payment data by
payment id with a merchant name instead
of merchant id
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7. CM Database Schema

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8. Payment Entity (1 of 2)

@Entity
public class Payment {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private double charge;
private double total;
private String goods;
private java.sql.Date dt;
@ManyToOne
@JoinColumn(name="merchantId")
private Merchant merchant;
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9. Payment Entity (2 of 2)

public Merchant getMerchant(){ return merchant; }
public void setMerchant(Merchant value){
merchant = value;
}
. . . . . . . .. . . . . . . .
public String getStringForPrint() {
String txt = "id = " + id + "; date = ";
SimpleDateFormat dtFrm = new SimpleDateFormat("dd.MM.yyyy");
txt += dtFrm.format(dt) + "; merchant = " + merchant.getName();
txt += "; total = " + total + "; charge = " + charge;
return txt;
}
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10. DAO & Service Interfaces

DAO & Service Interfaces
public interface PaymentDao {
public List<Payment> findByMerchantId(int id);
public double getPaymentSum();
public Payment findById(int id);
}
public interface PaymentService {
public List<Payment> findByMerchantId(int id);
public double getPaymentSum();
public Payment findById(int id);
}
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11. PaymentDaoImpl Class

public Payment findById(int id){
return em.find(Payment.class, id);
}
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12. Main Class

@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new
ClassPathXmlApplicationContext("beans.xml");
PaymentService paymentService =
context.getBean(PaymentService.class);
Payment p = paymentService.findById(2);
System.out.println(p.toString());
}
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13. Exercise: Read a Payment

• See P351ReadPayment project for the full
text
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14. Exercise: Group Payments

• Modify P237Grouping project using
entities from the previous project
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15. PaymentDaoImp Class

public List<Result> getTotalReport(){
String txt = "SELECT new com.bionic.edu.Result
(p.merchant.name, count(p), SUM(p.chargePayed))";
txt += "FROM Payment p GROUP BY p.merchant.name";
TypedQuery<Result> query = em.createQuery(txt,
Result.class);
return query.getResultList();
}
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16. Main Class

@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new
ClassPathXmlApplicationContext("beans.xml");
PaymentService paymentService =
context.getBean(PaymentService.class);
List<Result> list = paymentService.getTotalReport();
for(Result r: list)
System.out.format("%1$25s %2$3d %3$8.2f \n",
r.getName(), r.getCount(), r.getSum());
}
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17. Exercise: Group Payments

• See P352GroupPayment project for the
full text
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18. One-to-Many Mappings

• An entity instance can be related to multiple
instances of the other entities
• One-to-many relationships use the @OneToMany
annotation on the corresponding persistent
property or field
• The mappedBy element is needed to refer to the
attribute annotated by ManyToOne in the
corresponding entity:
@OneToMany(mappedBy=“attribute”)
• A one-to-many association needs to map the
collection of entities
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19. Exercise: Read a Merchant’s Payments

• Create a project for read payment data
that correspond to a merchant given by its
id
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20. CM Database Schema

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21. Merchant Entity

@Entity
public class Merchant {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
private double charge;
private int period;
private double total;
@OneToMany(mappedBy="merchant")
private Collection<Payment> payments;
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22. Main Class (1 of 2)

@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new
ClassPathXmlApplicationContext("beans.xml");
MerchantService merchantService =
context.getBean(MerchantService.class);
List<Merchant> list =
merchantService.getSortedByNeedToPay();
for(Merchant m: list){
System.out.println("=======================");
System.out.println(m.getName());
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23. Main Class (2 of 2)

System.out.println(" ");
Collection<Payment> payments = m.getPayments();
for (Payment p : payments) {
System.out.println(p.toString());
}
}
}
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24. Exercise: Read a Merchant’s Payments

• See P353MerchantPayments project for
the full text
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25. Many-to-Many Mappings

• The entity instances can be related to
multiple instances of each other
• Many-to-many relationships use the
@ManyToMany annotation on the
corresponding persistent property or field
• We must use a third table to associate the
two entity types (join table)
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26. @JoinTable Annotation

@JoinTable(name=“table name”,
joinColumns=@JoinColumn(name=“c1Id”),
inverseJoinColumns=
@JoinColumn(name=“c2Id”))
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27. Exercise: Read Customer’s Merchants

• Create a project to read data about
merchants whose goods were bought by a
given customer
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28. Customer Entity (1 of 2)

@Entity
public class Customer {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private java.sql.Date maturity;
private String ccType;
private String ccNo;
private String eMail;
private String address;
private String name;
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29. Customer Entity (2 of 2)

@ManyToMany
@JoinTable(name="Payment",
joinColumns=@JoinColumn(name="customerId"),
inverseJoinColumns=@JoinColumn(name="merchantId"
))
private Collection<Merchant> merchants;
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30. Main Class

@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new
ClassPathXmlApplicationContext("beans.xml");
CustomerService customerService =
context.getBean(CustomerService.class);
Customer customer = customerService.findById(1);
if (customer != null){
System.out.println(customer.toString());
Collection<Merchant> merchants = customer.getMerchants();
for (Merchant m : merchants) {
System.out.println(m.getName());}}}
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31. Exercise: Read Customer’s Merchants

• See P354CustAndMerch project for the full
text
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32. Lazy Relationships

• The fetch mode can be specified on any of
the four relationship mapping types:
@ManyToOne(fetch=FetchType.LAZY)
• Lazy loading can essential enhance
performance
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33. One-to-One Mappings

• Each entity instance is related to a single
instance of another entity
• One-to-one relationships use the
@OneToOne annotation on the
corresponding persistent property or field
• Example of One-To-One mapping:
Employee and ParkingSpace entities
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34. Embedded Objects (1 of 2)

• An embedded object is one that is dependent
on an entity for its identity
• It has no identity of its own, but is merely part
of the entity state
• The state of the embedded object is stored
with the rest of the entity state in the
database row, with no distinction between
the state in the Java entity and that in its
embedded object
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35. Embedded Objects (2 of 2)

• We can share the same embedded object
type with other entities that also have the
same internal representation
• Example: the home address of the
employee (contains Street, City, State,
ZipCode fields)
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36. Embedded Type

• An embedded type is marked as such by
adding the @Embeddable annotation to
the class definition
• Its fields and properties will be persistable
as part of an entity
• We might also want to define the access
type of the embeddable object so it is
accessed the same way regardless of
which entity it is embedded in
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37. Address Embedded Type

@Embeddable
@Access(AccessType.FIELD)
public class Address {
private String street;
private String city;
private String state;
private String zipCode;
// ...
}
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38. Using an Embedded Object

@Entity
public class Employee {
@Id
private int id;
private String name;
private long salary;
@Embedded
private Address address;
// ...
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39. Reuse of Embedded Type

• An Address class could be reused in both
Employee and Company entities
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40. Database Schema Generation

• Automatically generation the tables and
database schema for a persistence unit
can be done through the "eclipselink.ddlgeneration" persistence unit property
• The tables and constraints will be
generated for all of the classes defined in
that persistence unit.
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41. eclipselink.ddl-generation Values

• create-tables – if the table already exists
then it will not be dropped or replaced, the
existing table will be used
• drop-and-create-tables - will first drop the
existing table, and then create the new
table. Note that this will lose all of the data
in the tables when they are dropped!!
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42. Schema Generation Practice

• One of the complaints around schema
generation is that you can’t specify
everything that you need to be able to
finely tune the schema.
• There are too many differences between
databases and too many different settings
to try to put in options for every database
type.
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43. Schema Generation Practice

• If every database-tuning option were
exposed through JPA then we would end
up duplicating the features of Data
Definition Language (DDL) in an API
• The final schema will typically be tuned by
a database administrator or someone with
the appropriate level of database
experience.
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