Entity Relationships. Seven Relationship Types Four types of cardinality: –One-to-One –One-to-Many –Many-to-One –Many-to-Many Each relationship can be:

Entity Relationships

Seven Relationship Types

• Four types of cardinality:– One-to-One– One-to-Many– Many-to-One– Many-to-Many

• Each relationship can be:– Unidirectional– Bidirectional

• One-to-Many and Many-to-One bidirectional relationships are identical

Relationships

• In order to model real-world business concepts, entity beans must be capable of forming complex relationships– Examples:

•A Customer entity can have a one-to-one relationship with an Address entity

•A Customer entity can have a one-to-many relationship with a Phone entity

• Represent the navigability of the domain model

• Specified by applying annotations to related entity beans

One-to-One UnidirectionalRelationship

One-to-One Unidirectional Relationship

Customer+ getLastName() : String+ setLastName(In : String) : void

+ getFirstName() : String+ setFirstName(In : String) : void

+ getAddress() : Address+ setAddress(In : Address) : void

Address+ getStreet() : String+ setStreet(In : String) : void

+ getCity() : String+ setCity(In : String) : void

…

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One-to-One Unidirectional – Relational Database Schema

• CUSTOMER table contains a foreign key to the ADDRESS table

• ADDRESS table does not contain a foreign key to the CUSTOMER table

One-to-One Unidirectional – Entity Annotation

@Entitypublic class Customer implements java.io.Serializable {...private Address address;...@OneToOne(cascade={CascadeType.ALL})@JoinColumn(name=“ADDRESS_ID”)public Address getAddress() {

return address;}public void setAddress(Address addr) {

this.address = addr;}

One-to-One BidirectionalRelationship

One-to-One Bidirectional Relationship

Customer+ getLastName() : String+ setLastName(In : String) : void

+ getFirstName() : String+ setFirstName(In : String) : void

+ getCreditCard() : CreditCard+ setCreditCard(In : CreditCard) : void

CreditCard+ getNumber() : String+ setNumber(In : String) : void

+ getExpiration() : Date+ setExpiration(In : Date) : void

+ getCustomer() : Customer+ setCustomer(In : Customer) : void

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One-to-One Bidirectional – Relational Database Schema

• CUSTOMER table contains a foreign key to the CREDIT_CARD table

• CREDIT_CARD table does not contain a foreign key to the CUSTOMER table

• Bidirectional relationship established and maintained by the application programmer, not the persistence provider

One-to-One Bidirectional – Entity Annotation

@Entitypublic class Customer implements java.io.Serializable {...private CreditCard credCard;...@OneToOne(cascade={CascadeType.ALL})@JoinColumn(name=“CREDIT_CARD_ID”)public CreditCard getCreditCard() {

return credCard;}public void setCreditCard(CreditCard cc) {

this.credCard = cc;}

One-to-One Bidirectional – Entity Annotation

@Entitypublic class CreditCard implements java.io.Serializable {...private Customer customer;...@OneToOne(mappedBy=“creditCard”)public Customer getCustomer() {

return customer;}public void setCustomer(Customer cust) {

this.customer = cust;}

One-to-One Bidirectional – Establishing the Relationship

Customer cust = new Customer();CreditCard card = new CreditCard();cust.setCreditCard(card);card.setCustomer(cust);

One-to-One Bidirectional – Modifying the Relationship

Customer newCust = em.find(Customer.class, newCustId);

CreditCard card = oldCustomer.getCreditCard();

oldCustomer.setCreditCard(null);newCust.setCreditCard(card);card.setCustomer(newCust);

One-to-Many UnidirectionalRelationship

One-to-Many Unidirectional Relationship

Customer+ getLastName() : String+ setLastName(In : String) : void

+ getFirstName() : String+ setFirstName(In : String) : void

+ getPhones() : Collection<Phone>+ setPhones(In : Collection<Phone>)

: void

Phone+ getNumber() : String+ setNumber(In : String) : void

+ getType() : String+ setType(In : String) : void

*1

One-to-Many Unidirectional – Relational Database Schema

• Could be mapped with a join table or:

• PHONE table contains a foreign key to the CUSTOMER table (reverse pointer scenario)

One-to-Many Unidirectional – Entity Annotation@Entitypublic class Customer implements java.io.Serializable {...private Collection<Phone> phones =

new ArrayList<Phone>();...@OneToMany(cascade={CascadeType.ALL})@JoinColumn(name=“CUSTOMER_ID”)public Collection<Phone> getPhones() {

return phones;}public void setPhones(Collection<Phone> p) {

this.phones = p;}

One-to-Many Unidirectional – Establishing the Relationship

Customer cust = em.find(Customer.class, pk);Phone ph = new Phone(“617-333-3333”, 5);cust.getPhones().add(ph);

One-to-Many Unidirectional – Removing the Relationship

cust.getPhones().remove(ph);em.remove(ph);

One-to-Many Unidirectional – Join Table Mapping@Entitypublic class Customer implements java.io.Serializable {

...private Collection<Phone> phones =

new ArrayList<Phone>();...@OneToMany(cascade={CascadeType.ALL})@JoinTable(name=“CUSTOMER_PHONE”,

joinColumns={@JoinColumn(name=“CUSTOMER_ID”)}, inverseJoinColumns={@JoinColumn(name=“PHONE_ID”)})

public Collection<Phone> getPhones() {return phones;

}public void setPhones(Collection<Phone> p) {

this.phones = p;}

Many-to-One UnidirectionalRelationship

Many-to-One Unidirectional Relationship

Flight+ getNumber() : int+ setNumber(In : int) : void

+ getPlane() : Plane+ setPlane(In : Plane) : void

Plane+ getModel() : String+ setModel(In : String) : void

…1*

Many-to-One Unidirectional – Relational Database Schema

• FLIGHT table contains a foreign key to the PLANE table

Many-to-One Unidirectional – Entity Annotation

@Entitypublic class Flight implements java.io.Serializable {...private Plane plane;...@ManyToOne@JoinColumn(name=“PLANE_ID”)public Plane getPlane() {

return plane;}public void setPlane(Plane pl) {

this.plane = pl;}

One-to-Many BidirectionalRelationship

One-to-Many Bidirectional Relationship

Reservation+ getNumber() : int+ setNumber(In : int) : void

+ getRes() : Collection<Reservation>+ setRes(In : Collection<Reservation>)

: void

Flight

+ getDate() : Date+ setDate(In : Date) : void

+ getFlight() : Flight+ setFlight(In : Flight) : void

…

1*

One-to-Many Bidirectional – Relational Database Schema

• RESERVATION table contains a foreign key to the FLIGHT table

One-to-Many Bidirectional – Entity Annotation

@Entitypublic class Reservation implements java.io.Serializable {...private Flight flight;...@ManyToOne@JoinColumn(name=“FLIGHT_ID”)public Flight getFlight() {

return flight;}public void setFlight(Flight fl) {

this.flight = fl;}

One-to-Many Bidirectional – Entity Annotation@Entitypublic class Flight implements java.io.Serializable {...private Collection<Reservation> res =

new ArrayList<Reservation>();...@OneToMany(mappedBy=“flight”)public Collection<Reservation> getRes() {

return res;}public void setRes(Collection<Reservation> r) {

this.res = r;}

Many-to-Many BidirectionalRelationship

Many-to-Many Bidirectional Relationship

Reservation+ getName() : String+ setName(In : String) : void

+ getRes() : Collection<Reservation>+ setRes(In : Collection<Reservation>)

: void

Customer

+ getDate() : Date+ setDate(In : Date) : void

+ getCust() : Collection<Customer>+ setCust(In : Collection<Customer>)

: void

**

Many-to-Many Bidirectional – Relational Database Schema

• Use a RESERVATION_CUSTOMER association table

Many-to-Many Bidirectional – Entity Annotation@Entitypublic class Reservation implements

java.io.Serializable {...private Set<Customer> custs =

new HashSet<Customer>();...@ManyToMany@JoinTable(name=“RESERVATION_CUSTOMER”), joinColumns={@JoinColumn(name=“RESERVATION_ID”)},

inverseJoinColumns={@JoinColumn(name=“CUSTOMER_ID”)})public Set<Customer> getCust() {

return custs;}public void setCust(Set<Customer> cu) {

this.custs = cu;}

Many-to-Many Bidirectional – Entity Annotation@Entitypublic class Customer implements java.io.Serializable {...private Collection<Reservation> res =

new ArrayList<Reservation>();...@ManyToMany(mappedBy=“cust”)public Collection<Reservation> getRes() {

return res;}public void setRes(Collection<Reservation> r) {

this.res = r;}

Many-to-Many UnidirectionalRelationship

Many-to-Many Unidirectional Relationship

Reservation+ getLocation() : String+ setLocation(In : String) : void

PlaneSeat

+ getDate() : Date+ setDate(In : Date) : void

+ getSeats() : Collection<PlaneSeat>+ setSeats(In : Collection<PlaneSeat>)

: void

**

Many-to-Many Unidirectional – Relational Database Schema

• Use a RESERVATION_PLANESEAT association table

Many-to-Many Unidirectional – Entity Annotation@Entitypublic class Reservation implements

java.io.Serializable {...private Set<PlaneSeat> planeSeats =

new HashSet<PlaneSeat>();...@ManyToMany@JoinTable(name=“RESERVATION_PLANESEAT”), joinColumns={@JoinColumn(name=“RESERVATION_ID”)},

inverseJoinColumns={@JoinColumn(name=“PLANESEAT_ID”)})public Set<PlaneSeat> getSeats() {

return planeSeats;}public void setSeats(Set<PlaneSeats> ps) {

this.planeSeats = ps;}

Cascading

cascade

• Attribute of @OneToOne, @OneToMany, @ManyToOne, and @ManyToMany relationship annotations

@OneToOne(cascade={CascadeType.ALL})

Cascading

• When you preform an entity manager operation on an entity, you can automatically have the same operation performed on any relationship properties the entity may have

• Example: Customer entity has one-to-one relationship with Address entity and one-to-many relationship with Phone entity

Customer cust = new Customer();cust.setAddress(new Address());cust.getPhones().add(new Phone());// create all in one entity manager invocationentityManager.persist(cust);

Applying Cascading

• Cascading can be applied to a variety of entity manager operations using javax.persistence.CascadeType:

public enum CascadeType{ALL, PERSIST, MERGE, REMOVE, REFRESH

}

Cascade Types

• PERSIST– Inserts records of related objects

• MERGE– Inserts and updates records of related

objects• REMOVE

– Removes records of related objects• REFRESH

– Refreshes related object from the database

• ALL– Combination of all of the above

When to Use Cascading

• Not always applicable– Example: Do not want to remove a

related Flight when removing a Reservation

• Cascading simply a convenience tool for reducing the number of EntityManager API calls

Seven Relationship Types

• Four types of cardinality:– One-to-One– One-to-Many– Many-to-One– Many-to-Many

• Each relationship can be:– Unidirectional– Bidirectional

• One-to-Many and Many-to-One bidirectional relationships are identical