[FIXED] why One-To-Many association cannot be Bidirectional and it should just be Unidirectional?

Issue

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?

Solution

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 and relationships are defined in 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 your 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

Answered By - Harry Coder
Answer Checked By - Candace Johnson (JavaFixing Volunteer)