Hibernate Assosiations :Mapping Relations

Many to One Relation

Let us take the case of Employee and Company , its a typical case for many to one relation. Many employees can belong to a single Company.

Now this relation is unidirectional

@Entity
public class Employee
{

@Id
@GeneratedValue
@Column(name = "F_EMP_ID")
private int empId;

@ManyToOne(optional = true,fetch=FetchType.EAGER)
@JoinColumn
(name = "F_EMP_CMPNY_ID",referencedColumnName="F_CMPNY_ID")
private Company company;

private String name ;
.....
.....



getter and setters for all the values ...

}

@Id : shows its primary key

@GeneratedValue : asks hibernate to generate the primary keys

@Column : to give a java name for a corresponding DB field name

@ManyToOne : shows the many to one realtionship with field company
<Company>

optional : says if this field can be null for Employee row ( true is the default value)

fetch : whether to eagerly fetch the dependent rows (of company ) when fetching employee

(LAZY if the default value , we can change it to FetchType.EAGER)

@JoinColumn - how to join the column for the realtionship

name database field of the current entity which is used for joining

referencedColumnName data base field of the joining entity used for joining
If this field is not mentioned the primary key of the joing entity is taken hence in the above example we ned not mention this value

The other entity in the relationship

@Entity
public class Company
{

@Id
@GeneratedValue
@Column(name = "F_CMPNY_ID")
private int cmpnyId;

private String cmpnyName ;

.....

getter and setters for all the values ...
}


Now in the above example the mapping is complete .Now we can use the entitymanager to fetch any row of Emlpoye entity
(as showm in the previous blog)

entityManager.find(Employee.class, id) --> gives the corresponding Employee

(Note : This statement will come under the scope of @Transactional which manages the session)

Now we can use Employee.getCompany() -> which gives the coresponding Company value

Note:

This is possible because we used fetch=FetchType.EAGER while mentioning the manytoone relation.This ensure that when the request is made for Employee the corresponding Company is also fetched from the DB.

If this option(fetch=FetchType.EAGER) was not mentioned the default one would be used ,that is (fetch=FetchType.LAZY)

Lazy initilization commands hibernate that when Employee is fetched the corresponding , the coresponding Company is not fetched . This is called lazy initilization.Hibernate tries to fetch Company only when i first request is made to the company field in the Employee instance.


Now if this request for company is made outide the scope of @Transaction , that is when no session is open , hibernate gibes a error saying it could not Lazily initialize Company because of Lazy init.

Hence care should be taken to see to that the first request to the depending Entity(in this Company )in the case of FetchType.EAGER should be made inside a valid session.

There is an alternative to get all the dependent rows , when we use Fetch.LAZY . this is by explicitly initializing hibernate.

//include this in the service which gets employee

Employee e = entityManager.find(Employee.class, id);
//this ensures that corresponding company is also fetched along with employee
Hibernate.initialize(employee.getCompany());
//employee.getCompany() -- i sthe first request to Company
return e;



Now the above relation is Unidirectional , that is employee gets Company .Now what if we want to get all the employees in a Company.

We can achive this by making it Bidirectional

We will make small chages in Company Entity to make it bidirectional

@Entity
public class Company
{

@Id
@GeneratedValue
@Column(name = "F_CMPNY_ID")
private int cmpnyId;

private String cmpnyName ;

@OneToMany(mappedBy = "company" ,fetch =FetchType.EAGER)
private List<Employee> employees;
.....



getter and setters for all the values ...
}

@OneToMany - > indicates that the relationship is onetomany with List of employess , that is this company instance can have list of employees

mappedBy -> indicates that the other side is the owner of the relation.

That is a corresponding relation is defined in Employee entity and the

Employee entity had a member variable of Company by name company

fetch =FetchType.EAGER - > defines the type of fetch (Described above)


Now this relation is bidirectional that is when we fetch a Company insatnce we will get all the employees working for it by

Company c = entityManager.find(Company.class, id);

c.getEmployees(); - will return all the emplyees working in that company



One to One mapping


NO\ow to show a one2one mapping lets take a case of Address Entity

public class Address
{

@Id
@GeneratedValue
@Column(name = "F_ADDRS_ID")
private Long id;


.....

// Getters and Setters ...
}


@Entity
public class Company
{

@Id
@GeneratedValue
@Column(name = "F_CMPNY_ID")
private int cmpnyId;

private String cmpnyName ;

@OneToOne(optional = false, fetch = FetchType.EAGER)
@JoinColumn(name = "F_CMPNY_ID", referencedColumnName = "F_ADDRS_ID")
private Address address;


getter and setters for all the values ...
}


The above relation is similar to the above mentioned many to one relation , and all the attributes for many to one also apply to this.

This relation in Unidirectional and can be made bidirectional by declaring a corresponding relation on Address Entity.

Some Interesting notes on Hibernate

When a primitive value is mapped from a java class to db table column remeber to assign default values to these in the DB ,else when null is
inserted to the column ,and when we retrieve this its gives an error saying null canot be casted to primitive members.Hence for example if null
is present in the DB to retrieve this Wrapper classes such as Integer should be used instead of int.

Though we mention relation(involving primary key) in the classes using annotaions it is also helpful to mention the primary again with indertable and
updatebale equlas false , because in certain cases we need to retrive a row(or certain fileds only) in the db just based on this value and it can be '
hence mentioned in the HQL query directly .

// @Column(name = "F_PRODUCT_CATEGORY_ID" , insertable = false , updatable = false)
// private int pSampleId;\



Still to Follow .....

Many To Many Relation ..