The below definition is taken from Hibernate association mapping tutorial here
Each book is related to one publisher while one publisher may publish many books.
I'd represent the above definition the below way in Java
class Book
{
Publisher pub;
}
class Publisher
{
Set<Book> books;
}
Question 1. Now I am confused as to why the tutorial says The association from book to publisher is called a “many-to-one” association. I thought There is a "one-to-many" association between the publisher and the book would have been the right approach.
Question 2. Below is the hbm for Book. Why shouldn't we declare a one-to-many in the Publisher's hbm mapping. What determines the location for this relationship mapping?
<class name="Book" table="BOOK">
<many-to-one name="publisher" class="Publisher" column="PUBLISHER_ID" />
</class>
it depends on whether the relationship is bidirectional. See section 7.3.2 here http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html/collections.html#collections-mapping.
a bidirectional relationship is when you can go from both the parent to the child (i.e. Publisher -> Book) and from child to parent (i.e. Book -> Publisher). The parent to child is a one-to-many, while the child to parent is a many-to-one.
Note you do not need bidirectional relationships. The 'right approach' depends exactly on what your application needs. If you need to only go from pu.lisher to book, you only need a one to many. If you only need to go from books to publisher, that you just need the many to one. If you need both, you can have both (i.e. make the relationship bidirectional)/
The association from Book to Publisher is many-to-one because there can be many books per publisher. This is irrelevant to the that fact that Book is only aware of its own Publisher. The association is to aid Hibernate in understanding the underlying representation.
There is nothing wrong with also declaring a one-to-many relationship from Publisher to Book. You can have both present. It depends on your data model and how you plan on interacting with it.
Question 1: Actually both are correct. It depends on the direction of the relationship. Many books belong to one publisher results in a many to one relationship from the book side.
Question 2: If you are interested in a bi-directional relationship then you can actually place a one to many on one side and a many to one on the other. However, you have to be careful to use the mappedBy attribute to ensure that hibernate knows that its the same relationship being defined from both ends.
A relationship can be unidirectional, or bidirectional.
The way you designed it in your Java code, it's bidirectional. This means that the same physical relationship (materialized by the foreign key) is viewed as two logical relationships :
from the book point of view : several books are published by one publisher (ManyToOne)
from the publisher point of view : one publisher publishes several books (OneToMany)
The XML mapping you show in your question describes one side of the relationship. But if you want to make it bidirectional, you must also declare the other side. One of the sides is declared as the inverse of the other.
Hibernate gives you the choice : you could have a collection of books in the publisher class (unidirectional one to many), or a publisher reference in the book class (unidirectional many to one), or both (bidirectional one to many/many to one).
Read the documentation for more details.
Related
I am trying to build a Book app.
The entities are so-
The Entity User has a field called Role which specifies if this is an Author or Reviewer. Currently, I am using an Enum data member to differentiate if the User is an Author or Reviewer. One single instance of a User cannot be both.
A Book will have 1...* Authors- So a User entity which is an Author will have a many-to-many relation with entity Book
A Book will have 1...* Reviewers- So a User entity which is a Reviewer will have a many-to-many relation with entity Book
I was wondering how would I implement the User side - I preferably want one single collection of Books- If the User is an Author, this will contain a reference to the books he has authored. If it is a reviewer, it will contain references to the books on which he is a reviewer.
I am wondering if there is any construct in JPA/ Hibernate which can enable this.
I could always implement Author and Reviewer as different entities, but I still want to know the answer to such a situation which I presume must be quite commonly encountered.
I've seen a lot of domains designed with circular reference. The image below describes a simple example for understand the question: A company has employees and departments and a department has employees, who belong to a department.
Logically, the model allows a employee who works for a company be related with a department of another company - of course, hypothetically, if business logic validation fails.
In some more complex cases, and according to the business rules, I don´t see another way to model.
Is it acceptable?
Thanks in advance.
It is absolutely fine to have such kind of circular association.
Of course there are lots of pitfall that shoot your feet. However with extra cautious such kind of relationship is fact not always harmful.
Things that you need to pay extra attention includes:
Try to define the "owner" of the relationship. This is especially important if you are going to persist the entity
Make sure the owning relationships are not circular. For example, in your example, you can define Company owning the Company-Department and Company-Employee relationship, and Department owning the Department-Employee relationship
Make sure the relationship is consistent. For example, for the bi-directional relationship between Employee and Department, when you remove an Employee from a Department, make sure you remove the Department from Employee consistently.
Try to minimize the way to manage relationship. For example, instead of providing both Department#addEmployee(Employee) and Employee#addDepartment(Department), just provide Department#addEmployee(Employee). This should make your work of keeping consistency easier.
Still, if you can manage to make it uni-directional and non-circular, it is always easier to handle.
If you really want to avoid the situation you've described (where an employee "E" is associated with a department "D", but "D" and "E" are associated with different companies), you can try making your object model a tree, with no cycles:
Company --> Department --> Employee
There are trade-offs here. For example, this model doesn't directy handle Employees with no Department (although this could be papered over with a fake department like NoDepartment). This model may also require two "hops" to get from Employee to Company.
For a JPA project I got asked to implement the following situation:
A book can have many authors.
Also there is an UML that states that an author can have many books and a book can have many authors.
I am a beginner when it comes to this subject.
Do I need an many to many relation (with an intersection table)
or do I need a bidirectional one to many relation.
Also the bidirectional part of a one to many relation confuses me. If anybody could explain this with an example that would be nice.
The bidirectional associations allow you to navigate both sides of this relationship both in Object space as well as in JPA/Hibernate Query Language.
In your example, a database many-to-many association is mandatory (meaning you have a BOOK, an AUTHOR and a BOOK_AUTHOR link table). Hibernate offers two possibilities for mapping this relationship:
You can model this with two entities: Book and Author, each one having a #ManyToMany association to the other (one being the owning side while the other being the inverse one)
You can have three entities: Book, Author and BookAuthor and this time Book have a #OneToMany association to BookAuthor and Author has also a #OneToMany association to BookAuthor. This option allows you two map additional link table columns (BOOK_AUTHOR creation_time).
I have little bit confusing many to many relationship between 3 entities. And i want to know how can be my object model look like. I have three Entities, A,B,C and A<->B (M:N) and associate table between both, A and B, is linked with another associate table which make another 1:n relationship with third entity. I have never seen such relationship which make 1:n relationship with another associate table. For further information please have look on following diagram.
Uploaded Image link
If i talk about object model then i will say "INSTANCE_A" has many "INSTANCE_B" instance and vice versa but i do not know how can i summarize relationship for "INSTANCE_C".
Please also let me know whether definition of such relationship between all three entities is right ? i mean is there any problem in relationship design.
Thanks in advance
EDIT: All arrows denote (1:n or m:1) relationship
The data model is correct, but the object model for these tables can be kind of trucky. I'd do something like this:
One class for TBL_A, with a List attribute of TBL_B
One class for TBL_B, with a List attribute of TBL_A
One class for TBL_C_TBL_A_B, with and an attribute for TBL_B, TBL_A and TBL_C
Mapping that in an ORM framework can get funky.
This shall bring you into the right direction. Try to design a UML diagram, or ER should be ok too. Here is some paper with a Model and the corresponding Java-Code for this model http://www.csd.uoc.gr/~hy252/references/UML_for_Java_Programmers-Book.pdf. (Go to -> Class diagrams chapter).
I have a ManyToMany relationship between two classes, for instance class Customer and class Item. A customer can buy several items and an item can be bought by different customers. I need to store extra information in this relationship, for example the day when the item was bought. I wonder how is this usually modelled in JPA, cause I'm not sure how to express this in code. Do I have to create a new class to model all the attributes of the relationship and make a manyToMany relationship between the other classes or is a better way to do this?
Thanks
The recommended way is to create a new association class to store the needed attributes, and two one-to-many associations to the two parties involved.
I guess you will indeed have to create a new class for the relationship.
Like you said yourself, the correct way is to create a new class with the additional attributes.