why One-To-Many association cannot be Bidirectional and it should just be Unidirectional?-CodePudding

I'm trying to learn about JPA and Hibernate and I was trying to learn about some database terms.

In a video on youtube, the teacher (at 2:42) said:

One-To-Many is only Unidirectional.

and she said suppose thsese two class:

class Person {

    List<Address> addresses;

}


class Address {

    Person p;

}

and she said:

this is One-To-Many. but Address cannot have a collection of Person because if it has a collection of Person, it's going to be Many-To-Many.

but I think she is not right, because we can have these two:

thePerson.getAddresses().get(i);

and

anAddress.getPerson();

When we can have these two statements, then it is Bidirectional. Then why she said it can be just Unidirectional?

What is Bidirectional's exact definition with which she came to such a conclusion?

CodePudding user response：

First of all, any relationship can be unidirectional or bidirectional, it's all depend on how you want to display or retrieve your data, and your business rules.

When we can have these two statements, then it is Bidirectional. Then why she said it can be just Unidirectional?

There is also a big mismatch between the object oriented approach of ORMs like Hibernate and how tables are and relationships are defined int a relational database. She can be wright, because, in the point of view of a database, relationships are by default unidirectional.

The difference between unidirectional and bidirectional is defined by the fact that you can access the records of the other side of your relationship from where you are.

What is Bidirectional's exact definition with which she came to such a conclusion?

In your example we can interpret you relationship like this:

Unidirectional: you can get all your addresses from person, but not the inverse

 class Person {

     @OneToMany
     List<Address> addresses;

 }

 class Address {

     // Here you don't add the ManyToOne
     Person p;
 }

Bidirectional: you can get all your addresses from person and get a person from an address

 class Person {

     @OneToMany
     List<Address> addresses;
 }

 class Address {

     @ManyToOne
     Person p;
 }

Take a look at those links below:

Difference between unidirectional and bidirectional relational relationship

https://www.baeldung.com/spring-data-rest-relationships

What is Object/Relational mismatch

CodePudding user response：

You are correct, she's not right. All kinds of relationships can de unidirectional or bidirectional.

On the child side, you have to annotate the field with @JoinColumn.

On the parent side, you have to use the property mappedBy of @OneToMany annotation.

Example:

@OneToMany(mappedBy = "user")
private List<Address> addresses;

@ManyToOne
@JoinColumn
private User user;

Here's a good lecture about it.

https://vladmihalcea.com/the-best-way-to-map-a-onetomany-association-with-jpa-and-hibernate/