How to make sure country is not added twice in mysql database in hibernate using spring mvc one to many relationship? - spring

Question

I have two entity first one is Student and the other one is an address. It is one too many relations ie one address can have many students. Now I have is a registration page. When I first register a student say with country name united states, it is saved in database giving primary id as 1 to united states and correspondingly gives correct id in student's database. But when I again try to register the next student with different information but the same country in my case united states it gives me a new primary key for the same country. But as this one is, one to many relationships I am thinking if there is anything in hibernate that maps to the same id in address database, therefore, I will only have one value of the united states in the address database. I need to have only a single entry of the united states a database. What is the appropraite way of doing need? Thank you
This one is Address table
#Entity
#Table(name = "tbl_address")
public class Address {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "address_id")
private int addressId;
private String country;
#OneToMany(targetEntity = Student.class, mappedBy = "address")
private List<Student> student;
This one is Student table
#Entity
#Table(name = "tbl_student")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "student_id")
private int studentId;
#Column(name = "first_Name")
private String firstName;
#Column(name = "second_Name")
private String secondName;
private String email;
#Column(name = "mobile_no")
private float mobileNo;
#DateTimeFormat(pattern = "yyyy-MM-dd")
#Temporal(TemporalType.DATE)
private Date dob;
private String gender;
#ManyToOne(cascade = {CascadeType.MERGE , CascadeType.ALL} )
#JoinColumn(name = "address_id")
private Address address;
}
This one is just the implementation in StudentRepositoryImpl class
#Override
public void saveUserInfo(Student user) {
Session session = HibernateUtil.getSession(sessionFactory);
session.save(user);
}
#Override
public void saveAddressInfo(Address address) {
// TODO Auto-generated method stub
Session session = HibernateUtil.getSession(sessionFactory);
session.save(address);
}

Answer 1

First option. On UI you should have a list of available countries (so I expect you already have populated country table in database). So UI will display to users available country names, but on backend side you will operate with countryId.
Second option. In case on UI you want users insert any string as country name, and if you want to register country on any new provided country name. For that you need to have unique index in country table on country name (it's up to you to decide whether it will be case insensitive or not).
Before saving student entity, you should fetch available country by it's name. if such one exist - use countryId with saving studend. if not exist - create a new country entity, and use generated countryId in studend entity.
Second option is risky in that way users could provide any values for country names, and probably with typo, so the first option is preferable.

Answer 2

Have a separate table for Country (That would be called master data). Then the Address entity would be something like this:
#Entity
#Table(name = "tbl_address")
public class Address {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "address_id")
private int addressId;
#ManyToOne
private Country country;
#OneToMany(targetEntity = Student.class, mappedBy = "address")
private List<Student> student;
You can get the list of students doing a join query with Country and Address tables.
Hope this is helpful