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3. Java Persistence API 4. Entity Relationships

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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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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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CM Database Schema

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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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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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CM Database Schema

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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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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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DAO & Service Interfaces public interface PaymentDao { public List findByMerchantId(int id); public double getPaymentSum(); public Payment findById(int id); } public interface PaymentService { public List findByMerchantId(int id); public double getPaymentSum(); public Payment findById(int id); }

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

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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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Exercise: Read a Payment See P351ReadPayment project for the full text

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Exercise: Group Payments Modify P237Grouping project using entities from the previous project

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PaymentDaoImp Class public List 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 query = em.createQuery(txt, Result.class); return query.getResultList(); }

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Main Class @SuppressWarnings("resource") public static void main(String[] args) { ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml"); PaymentService paymentService = context.getBean(PaymentService.class); List 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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Exercise: Group Payments See P352GroupPayment project for the full text

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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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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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CM Database Schema

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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 payments;

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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 list = merchantService.getSortedByNeedToPay(); for(Merchant m: list){ System.out.println("======================="); System.out.println(m.getName());

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Main Class (2 of 2) System.out.println(" "); Collection payments = m.getPayments(); for (Payment p : payments) { System.out.println(p.toString()); } } }

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Exercise: Read a Merchant’s Payments See P353MerchantPayments project for the full text

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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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@JoinTable Annotation @JoinTable(name=“table name”, joinColumns=@JoinColumn(name=“c1Id”), inverseJoinColumns= @JoinColumn(name=“c2Id”))

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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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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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Customer Entity (2 of 2) @ManyToMany @JoinTable(name="Payment", joinColumns=@JoinColumn(name="customerId"), inverseJoinColumns=@JoinColumn(name="merchantId")) private Collection merchants;

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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 merchants = customer.getMerchants(); for (Merchant m : merchants) { System.out.println(m.getName());}}}

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Exercise: Read Customer’s Merchants See P354CustAndMerch project for the full text

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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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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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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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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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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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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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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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Reuse of Embedded Type An Address class could be reused in both Employee and Company entities

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

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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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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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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.