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One-to-Many Unidirectional Relationship

One-to-Many Unidirectional Relationship

Learn how to implement one-to-many relationships and about the orphan removal attribute.

We'll cover the following...

To show the one-to-many relationship, we will model the case where many players can register for a tournament. We will create a tournament table and a registration table to model this relationship.

Unidirectional one-to-many relationship means that only one side maintains the relationship details. So given a Tournament entity, we can find the Registrations but we cannot find the Tournament details from a Registration entity.

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One-to-Many unidirectional relationship
One-to-Many unidirectional relationship

For the sake of organization, we will place all files for the one-to-many relationship example in a separate package. Create a new package named onetomany.uni inside io.datajek.databaserelationships. Copy the DatabaseRelationshipsApplication.java file from the main package.

Creating entities

To model the one-to-many relationship, we will define a Tournament class with three fields: id, name, and location. The id field is the primary key. We can also save other details like the dates in which the tournament takes place, the type of surface on which it will be played, and the number or rounds etc. The code of the Tournament class with constructors and getter and setter methods is shown below:

@Entity

public class Tournament {

    @Id

	@GeneratedValue(strategy = GenerationType.IDENTITY)

	private int id;



	private String name;

	private String location;

	

	public Tournament() {		

	}



	public Tournament(String name, String location) {

		super();

		this.name = name;

		this.location = location;

	}

	

	public int getId() {

		return id;

	}



	public void setId(int id) {

		this.id = id;

	}



	public String getName() {

		return name;

	}



	public void setName(String name) {

		this.name = name;

	}



	public String getLocation() {

		return location;

	}



	public void setLocation(String location) {

		this.location = location;

	}



	@Override

	public String toString() {

		return "Tournament [id=" + id + ", name=" + name + ", location=" + location + "]";

	}

}
Tournament entity

Next, define the Registration class with just one field, id, for now. The id field is the primary key for the table. We will add more fields later.

The Registration class can store information about the registration date, the type of match (single/ doubles) for which the player registers, and the rank assigned to the player (seed) etc.

@Entity

public class Registration {

    @Id

    @GeneratedValue(strategy=GenerationType.IDENTITY)

    private int id;



    public Registration() {		

    }



    public int getId() {

      return id;

    }



    public void setId(int id) {

      this.id = id;

    }



    @Override

    public String toString() {

      return "Registration [id=" + id + "]";

    }

}
Registration entity

One to many relationship

Since a player registers for a tournament, a registration object should be associated with a player object.

We will update the Tournament class to show the registrations. Since a tournament can have multiple registrations, we will add a List of Registrations as a new field.

public class Tournament {

    //...

    private List<Registration> registrations = new ArrayList<>();

    //...



    //update constructor, generate getter and setter methods , update toString()

    public Tournament(String name, String location, List<Registration> registrations) {

        super();

        this.name = name;

        this.location = location;

        this.registrations = registrations;

    }



    public List<Registration> getRegistrations() {

    return registrations;

    }



    public void setRegistrations(List<Registration> registrations) {

    this.registrations = registrations;

    }



    @Override

    public String toString() {

    return "Tournament [id=" + id + ", name=" + name + ", location=" + location + ", registrations=" + registrations

    + "]";

    }

}
List of Registrations in Tournament class

@OneToMany

The Tournament class has a one-to-many relationship with the Registration class as one tournament can have multiple registrations. This can be modelled by the @OneToMany annotation. In a one-to-many relationship, the primary key ...