Develop Reference

Hibernate: How to distinguish two uni-directional relationship and one bi-directional relationship?

in this answer, the author said:
Take an example of two entities mapped without declaring a owning
side:
#Entity
#Table(name="PERSONS")
public class Person {
#OneToMany
private List<IdDocument> idDocuments;
}
#Entity
#Table(name="ID_DOCUMENTS")
public class IdDocument {
#ManyToOne
private Person person;
}
From a OO point of view this mapping defines not one bi-directional
relation, but two separate uni-directional relations.
1.My first question is:
Why it is not a bi-directional relation?
Docs Oracle:
In a bidirectional relationship, each entity has a relationship field
or property that refers to the other entity. Through the relationship
field or property, an entity class’s code can access its related
object
in the above code, both class have a relationship field that refers to the other entity. then why it is not a bi-directional relation and these are two uni-directional relations?
2.My second question is: what is the difference between two uni-directional relations and one bi-directional relation? aren't they same thing?

(1) No ideas why the author said it is not a bi-directional relationship.
From the hibernate documentation , it also mentions that a similar mapping is a bidirectional which both involved entities can navigate to each other.
In this example, given a Person , we can get its IdDocument by its idDocuments field. And given an IdDocument , we can get its Person by its person field. So it is a bi-directional.
(2) I am not sure too. To me, whenever both entities in a relationship can navigate to each other , it is a bi-directional relationship. If only one entity can navigate to another but not vice versa , it is a unidirectional relationship.
And you can find the equivalent unidirectional case of the example the I mentioned above in this for comparing their differences.

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

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?

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

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/

What is #OneToOne mapping in spring JPA?

I am new to Spring data jpa and trying to understand #OneToOne mapping.
Let's say I have a Employee entity and a Company Entity, If I want to map these 2 entities, then I can use one to one mapping on Employee entity which means one employee can belong to one company only.
Is this understanding wrong?
If one employee belongs to one company(lets say XYZ) then the company(XYZ) cannot be mapped to a different employee?
I have read few posts but not completely understood.

#OneToOne represents that there is only one Object of Entity related to the other Entity
if we have Employee and Passport Entity so only One Passport related to One Employee
and for sure one One Object of Employee related to One Object from Passport
#Entity
Public class Employee
{
#OneToOne
private Passport passport;
}
so from Employee i can get his Passport
#Entity
Public class Passport
{
#OneToOne
private Employee employee;
}
and from Passport i can get The Employee.

Mapping is nothing but defining the relationship between two entities/objects and it is just like stating 5>4, 10=10, 6<8. The numbers here (5,4,10,6 and 8) are the entities and the symbols (>, = and <) are the relationships/mapping between them.
We do the same with mappings in hibernate. Note down the two entities and put in between them the relationship (mapping) them in the way it makes more sense.
Father OneToMany Child (Father can be One, To Many child/children)
Child ManyToOne Father (Child/Children can be Many, To One Father)
Employee ManyToOne Company (Employee can be Many, To One Company)
Company OneToMany Employees (Company can be One, To Many Employee(s))
Address OneToOne Employee (Address can be One, To One Employee)
Employee OneToOne Address (Employee can be One, To One Address)
The relationship should make sense. Which means it should make sense when you look at the relationship from each of the both sides of the relationships (One child Many Fathers doesnt make sense but One Father many child/children does)

Say an employee can be mapped to a Company that is one to one mapping keeping Employee as owner of the relationship. Whereas if you view Company as owner of the relationship, that it is One to Many.
Case 1 : Employee as Owner
#Entity
Public class Employee
{
#ManyToOne
private Company company;
......
}
#Entity
Public class Company
{
#OneToMany(mappedBy="company") \\ mappedBy is used to say that Employee is owner and
\\it should match variable name company
private List<Employee> employee;
......
}

Yes, you are correct.
If you want the Company to have multiple Employees, then you want a ManyToOne relationship between Employee and Company, and a OneToMany relationship between the Company and employee.

Actually, in this case, You must use one to many relationship.
you can simply use the #ManytoOne annotation in Company Entity that related to Employee Entity.
Specifies a single-valued association to another entity class that has many-to-one multiplicity. It is not normally necessary to specify the target entity explicitly since it can usually be inferred from the type of the object being referenced. If the relationship is bidirectional, the non-owning OneToMany entity side must used the mappedBy element to specify the relationship field or property of the entity that is the owner of the relationship.
Visit https://en.wikibooks.org/wiki/Java_Persistence/ManyToOne for more information and samples.

Understanding bidirectional relationship from Hibernate documentation

I am going through the Hibernate documentation for bidirectional relationship, in the doc it says that:
Example 7.21. Bidirectional one to many with many to one side as association owner
#Entity
public class Troop {
#OneToMany(mappedBy="troop")
public Set<Soldier> getSoldiers() {
...
}
#Entity
public class Soldier {
#ManyToOne
#JoinColumn(name="troop_fk")
public Troop getTroop() {
...
}
Troop has a bidirectional one to many relationship with Soldier
through the troop property. You don't have to (must not) define any
physical mapping in the mappedBy side.
To map a bidirectional one to many, with the one-to-many side as the
owning side, you have to remove the mappedBy element and set the many
to one #JoinColumn as insertable and updatable to false. This solution
is not optimized and will produce additional UPDATE statements.
Example 7.22. Bidirectional association with one to many side as owner
#Entity
public class Troop {
#OneToMany
#JoinColumn(name="troop_fk") //we need to duplicate the physical information
public Set<Soldier> getSoldiers() {
...
}
#Entity
public class Soldier {
#ManyToOne
#JoinColumn(name="troop_fk", insertable=false, updatable=false)
public Troop getTroop() {
...
}
I am finding difficulty in understanding this as I am new to Hibernate.
1) What it means when the doc says:
You don't have to (must not) define any physical mapping in the mappedBy side.
2) #JoinColumn in 7.22 has same value (troop_fk) for name attribute. Can we specify different values? What is the advantage & disadvantages of setting insertable=false, updatable=false here?
Can someone please explain?

It's a bidirectional association. So, if a soldier balongs to a troop, the troop contains the soldier. These are just the two ways of saying the same thing.
In Soldier, you tell how the association is represented in the database: using a join column named troop_fk:
#JoinColumn(name="troop_fk")
public Troop getTroop() {
So, repeating that same information on the other side of this bidirectional association is redundant. You must not do it. By saying
#OneToMany(mappedBy="troop")
public Set<Soldier> getSoldiers() {
you're telling Hibernate that getSoldiers() is the inverse side of a bidirectional association, and that the way this association is mapped can be found on the Soldier.troop property.
Regarding your second question. Once again, the goal is to define a single, but bidirectional association. You don't need two different foreign keys to map a single association. So specifying a different name for the join column would make no sense: it would create a different, unidirectional association.
This way of doing is an ugly hack that, AFAIK, is not supported by the JPA spec. The JPA spec mandates that the owner side of a bidirectional OneToMany association is the many side. In fact it creates two unidirectional associations mapped the same way, and tells Hibernate (using insertable = false and updatable = false) to ignore one of them when saving the entity. It will populate soldier.troop when reading a soldier from the database, but whatever you put into soldier.troop will be ignored when saving the soldier. You should avoid this way of doing, IMHO.

Hibernate : Difference between # Embedded annotation technique and #OneToOne annotation Technique

What is the difference between #Embedded annotation technique and #OneToOne annotation technique because in Embedded the java class contain "Has a" relationship in class and with the help of #Embedded annotation we persist the has a object in database. and in OneToOne relationship we also persist the has a object in database.

#OneToOne is for mapping two DB tables that are related with a one to one relationship. For example a Customer might always have one record in a Name table.
Alternatively if those name fields are on the Customer table (not in a separate table) then you might want an #embedded. On the face of it you could just add the name fields as standard attributes to the Customer entity but it can be useful if those same columns appear on multiple tables (for example you might have the name columns on a Supplier table).

Its the difference between composition and aggregation. #Embedded objects are always managed within the lifecycle of their parents. If the parent is updated or deleted, they are updated or deleted as well. #OneToOne objects may mimic composition via the cascadeType option of their #Join annotation, but by default they are aggregated, aka their lifecycle is separate from that of their parent objects.

#Embedded is used with Value Objects (Objects which have a meaning only when attached to an Object) whereas one to one mapping is between two objects having their own existence and meaning.
For e.g.
Value Object and #Embedded: If we have a User class and this class has an address Object in it, it can be considered as a value object as the address alone does not have any significance until unless associated with a user. Here address object can be annotated with #Embedded.
One to One mapping and #OneToOne: If we have a User class and this class has a 'Father' Object or a 'Mother' object, we would want to annotate the 'Father' or 'Mother' instance as #OneToOne as 'Father' or 'Mother' have their own meaning and existence and are not Value objects to User class.
A closely related difference is between #OneToMany and #ElementCollection. Both are used to save instance variables of Collection type in Java class. The difference being, #ElementCollection is to be used when the elements of Collection being saved are Value Objects whereas #OneToMany is used when the elments and object have well defined meaning and existence.

Use #OneToOne, only if fields can be reused. Otherwise, go for #Embeddable.
A quote from Beginning Hibernte, 3rd Edition:
There is nothing intrinsically wrong with mapping a one-to-one association between two entities where one is not
a component of (i.e., embedded into) the other. The relationship is often somewhat suspect, however. You should
give some thought to using the embedded technique described previously before using the #OneToOne annotation.
#Embeddable:
If the fields in an entity (X) are contained within the same table as another entity (Y), then entity X is called "component" in hibernate terms or "embedded" in JPA terms. In any case, JPA or hibernate do not allow to use 2nd table to store such embedded entities.
Generally, we think of normalizing a table when data is being reused by more than one table. Example: A Customer (id, name, street, city, pin, landmark) can be normalized into Customer(id, name) and CustomerAddress(cust_id, street, city, pin, landmark). In this case, we can reuse CustomerAddress by linking the same using cust_id with other tables. But if this reuse is not required in your application, then we can just keep all columns in one table.
So, a thumb rule is,
If reuse -> #OneToOne,
If no reuse -> #Embeddable

#Embedded is typically to represent a composite primary key as an embeddable class:
#Entity
public class Project {
#EmbeddedId ProjectId id;
:
}
#Embeddable
Class ProjectId {
int departmentId;
long projectId;
}
The primary key fields are defined in an embeddable class. The entity contains a single primary key field that is annotated with #EmbeddedId and contains an instance of that embeddable class. When using this form a separate ID class is not defined because the embeddable class itself can represent complete primary key values.
#OneToOne is for mapping two DB tables that are related with a one to one relationship. #Id will be the primary key.