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Map an entity based on one of two different attributes (either one or the other)

I'm developing a web app using Java, Spring and JPA. The use case I would like to discuss with you is quite simple, it just consists of two entities, Wallet and Transaction. In particular a Wallet can have two kind of transactions, incoming and outgoing. Let's see the structure of these two classes.
#Entity
public class Wallet{
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long walletId;
#OneToMany(mappedBy="fromWallet", orphanRemoval = true)
private List<Transaction> outgoingTransactions;
#OneToMany(mappedBy="toWallet", orphanRemoval = true)
private List<Transaction> incomingTransactions;
....
}
public class Transaction{
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long transactionId;
#ManyToOne
#JoinColumn(name = "wallet_id_from")
private Wallet fromWallet;
#ManyToOne
#JoinColumn(name = "to_wallet_id")
private Wallet toWallet;
....
}
This is what my code looks like right now, and it works perfectly. My question is, how can I change the code so that instead of having two different lists I would have just one with both incoming and outgoing transactions? So what I would like is something like this:
#Entity
public class Wallet{
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long walletId;
#OneToMany(mappedBy="{'fromWallet' or 'toWallet'}", orphanRemoval = true)
private List<Transaction> transactions;
....
}
Is it possible to do something like this? Or I must stick to the current solution (two different list)?

I don't think the situation you are describing can be achieved with JPA tools, but you can of course do something like
Remodel your database and merge both join columns so that you end up with a "classical" OneToMany relationship with the downside that you lost the information if the wallet was from or to.
Create a getTransactions method in Wallet that simply returns a merged immutable list of outgoingTransactions and incomingTransactions without having the downside of 1.

Obtain column data from One-To-Many join relation in Spring data JPA

Suppose I have two database tables, Product and ProductDetails.
create table Product
{
product_id int not null,
product_name varchar(100) not null,
PRIMARY KEY (product_id)
}
create table ProductDetails
{
detail_id int not null,
product_id int not null,
description varchar(100) not null,
PRIMARY KEY (detail_id,product_id),
FOREIGN KEY (product_id) REFERENCES Product(product_id)
}
Each product can have multiple product detail entries, but each product detail can only belong to one product. In SQL, I want to be able to retrieve each product detail but with the product name as well, and I would do that with a join statement.
select p.product_id,pd.detail_id,p.product_name,pd.description
from Product p join ProductDetails pd on p.product_id=pd.product_id
Now I need to have that concept in Spring data JPA form. My current understanding is the following:
#Table(name = "Product")
public class ProductClass
{
private int productID;
private String productName;
}
#Table(name = "ProductDetails")
public class ProductDetailsClass
{
private int detailID;
private int productID;
// this is the part I don't know how to set. #OneToMany? #ManyToOne? #JoinTable? #JoinColumn?
private String productName;
private String description;
}
(I didn't include any attributes such as #Id to keep the code minimal)
What do I need to write to get this private String productName; working?
My research on the #JoinTable and #OneToMany and other attributes just confuses me more.
P.S. This is a legacy Java program I inherited. The private String productName; part wasn't in the original code, but now I need the ProductDetails class to have the productName available.
P.P.S. I want to have a clear understanding of what I'm doing before trying anything and deploying. This is a legacy program deployed to production, and from what I understand, any code changes here can change the database structure as well, and no amount of money is enough to make me want to restore the Java program, the Spring Framework, the Apache server and MySQL database to a working order if anything catastrophic happens. Also I don't really have a development environment to test this. Help...

You research already goes in the right direction: You would need a #OneToMany relationship. The best descriptions for Hibernate has Vlad Mihalcea. On his webpage you could also find a good explanation of those relationships: The best way to map a #OneToMany relationship with JPA and Hibernate.
Firstly, you would have to create the entities correctly (an entity is represented by a table in a relational database).
Unidirectional (#OneToMany)
#Entity
#Table(name = "product")
public class Product
{
#Id
#GeneratedValue
private Long productID;
private String productName;
#OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
private List<ProductDetail> productDetails;
//Constructors, getters and setters...
}
#Entity
#Table(name = "product_details")
public class ProductDetail
{
#Id
#GeneratedValue
private Long detailID;
private String description;
//Constructors, getters and setters...
}
This is based on a unidirectional relationship. Therefore, each Product knows all the allocated ProductDetails. But the ProductDetails do not have a link to its Products. However, this unidirectional implementation is not recommended. It results in an increase of the size of the database, even its optimisation with #JoinColumn is not ideal because of more SQL calls.
Unidirectional (#ManyToOne)
#Entity
#Table(name = "product")
public class Product
{
#Id
#GeneratedValue
private Long productID;
private String productName;
//Constructors, getters and setters...
}
#Entity
#Table(name = "product_details")
public class ProductDetail
{
#Id
#GeneratedValue
private Long detailID;
private String description;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = product_id)
private Product product;
//Constructors, getters and setters...
}
In this unidirectional relationship only the ProductDetails know which Product is assigned to them. Consider this for a huge number of ProductDetail objects for each Product.
The #JoinColumn annotation specifies the name of the column of the table product_details in which the foreign key to the Product (its id) is saved. It also works without but it is more efficient with this annotation.
Bidirectional (#OneToMany and #ManyToOne)
#Entity
#Table(name = "product")
public class Product
{
#Id
#GeneratedValue
private Long productID;
private String productName;
#OneToMany(mappedBy = "product", cascade = CascadeType.ALL, orphanRemoval = true)
private List<ProductDetail> productDetails;
//Constructors, add, remove method, getters and setters...
}
#Entity
#Table(name = "product_details")
public class ProductDetail
{
#Id
#GeneratedValue
private Long detailID;
private String description;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = product_id)
private Product product;
//Constructors, getters and setters...
}
With a bidirectional relationship objects of both sides (Product and ProductDetail) know which other objects got assigned to them.
But according to Vlad Mihalcea, this should not be used if too many ProductDetails exist per Product.
Also remember to implement proper add and remove methods for list entries (see article again, otherwise weird exceptions).
Miscellaneous
With the cascading, changes in a Product also get applied to its ProductDetails. OrphanRemoval avoids having ProductDetails without a Product.
Product product = new Product("Interesting Product");
product.getProductDetails().add(
new ProductDetails("Funny description")
);
product.getProductDetails().add(
new ProductDetails("Different description")
);
entityManager.persist(product);
Often the question about the correct equals and hashCode methods is a complex puzzle in your head. Especially for bidirectional relationships but also in other situations relying on a database connection it is recommendable to implement them quite simply as described by Vlad.
It is good practice to use objects for primitive data types as well. This gives you the option to retrieve a proper null when calling the getter.
Avoiding eager fetching should be quite clear...
When you now try to retrieve a Product out of the database, the object automatically has a list of all the ProductDetails assigned to it. To achieve this, JPA repositories in Spring could be used. Simple methods do not have to be implemented. When you have the need to customise the functionality more, have a look at this article by Baeldung.

How to stop Hibernate from eagerly fetching a relationship when it is mapped using a column (referencedColumnName) different than the primary key?

I'm mapping a relationship that does not use the entity's primary key. Using "referencedColumnName" with a column different than the primary key causes hibernate to eagerly fetch the association, by issuing an extra select, even when it's tagged with FetchType.LAZY.
My goal is to make it behave like a regular mapping, meaning it wouldn't issue an extra query every time I need to query the main entity.
I have already tried using #LazyToOne(LazyToOneOption.NO_PROXY), which sorts out the problem, but it does not operate well with Jackson's (JSON parsing library) module "jackson-datatype-hibernate5", which skips hibernate lazy proxies when serializing the results.
Here is a scenario almost like the one I have that causes the problem:
Entities:
#Entity(name = "Book")
#Table(name = "book")
public class Book
implements Serializable {
#Id
#GeneratedValue
private Long id;
private String title;
private String author;
#NaturalId
private String isbn;
//Getters and setters omitted for brevity
}
#Entity(name = "Publication")
#Table(name = "publication")
public class Publication {
#Id
#GeneratedValue
private Long id;
private String publisher;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(
name = "isbn",
referencedColumnName = "isbn"
)
private Book book;
#Column(
name = "price_in_cents",
nullable = false
)
private Integer priceCents;
private String currency;
//Getters and setters omitted for brevity
}
Repository (Spring-Data, but you could try directly with the EntityManager):
#Repository
public interface PublicationRepository extends JpaReadRepository <Publication, Long>
{
#Query ("SELECT d FROM Publication d WHERE d.publisher = ?1 ")
Optional <Publication> findByPublisher (String isbn);
}
Thanks

The only way to achieve what you are looking for is by moving the annotatation #Id to the isbn property.
You can leave the #GeneratedValue on the autoincrement property.
Notes:
1 - Make sure that your equals/hc are following the OID(Object ID) on your domain case the "NaturalId" ISBN.
2 - It will be good to ensure if possible on DB level that your natural ID has unique contraint on it.

JPA Hibernate :: Inheritance of an entity, with additional OneToMany Lists

I'm using JPA Hibernate/Spring boot to build a web server with MySQL database, and I'm trying to extend a POJO Entity that looks like this, with additional OneToMany Lists.
#Entity
#Table(name="user")
public class User {
#Id
#GeneratedValue
private Integer id;
#Column(nullable=false)
private String name;
....Constructors, getters and setters....
}
with this basic user entity, I just wanna make a UserInfo entity with additional information about the user's careers.
#Entity
public class UserInfo extends User {
#OneToMany(cascade= CascadeType.ALL, fetch= FetchType.EAGER)
#JoinColumn(name="user_id", referencedColumnName = "id")
private List<Career> careers;
....Constructors, getters, setters......
}
And I'm quite confused which inheritance strategy I should choose. I don't think its necessary to make another column or table for this.
Or should I just query twice..?
I'm kinda new to JPA so not sure which is considered as the best practice or design..
Edit:
This is how Career entity looks like. Just in case..
#Entity
#Table(name="career")
public class Career {
#Id
#GeneratedValue
private Integer id;
#Column(nullable=false)
private Integer user_id;
#Column(nullable=false)
private String name;
#Column(nullable=false)
private String description;
....Constructors, getters and setters....
}
Since extending User table was meaningless(just in my case), I changed the User class like this.
#Table(name="user")
public class User {
#Id
#GeneratedValue
private Integer id;
#Column(nullable=false)
private String name;
#OneToMany(fetch= FetchType.LAZY)
#JoinColumn(name="user_id", referencedColumnName = "id")
private List<Career> careers;
....Constructors, getters, setters......
}
Now I'm trying this with Spring Data JPA, and when I try to show the list of Users with their Careers, it is now querying more than 40 times taking about a minute to show the result.
Is this the N+1 problem..? how can I solve this?

In my opinion the error lies within the model itself. Why should UserInfo extend User? I cannot imagine which attributes or methods the UserInfo should inherit from a User. Typical inheritances would be "Developer" or "Administrator".
Why don't you add UserInfo as a 1:1 relation in your User entity? Another option is to omit UserInfo and put the Careers as a 1:n relation right into your User.
To prevent possible n+1 issues on a growing number of Careers you might want to change the fetch mode. See below
#OneToMany(fetch=FetchType.LAZY,mappedBy="user")
#Fetch(FetchMode.SUBSELECT)
private Set<Career> careers = new HashSet<>();

Why in this Hibernate mapping it is used #ManyToOne instead #OneToOne?

I am absolutly new in Hibernate development and I have the following problem.
I have 2 entity classes that maps 2 DB tables:
1) The first entity class (the main one) is named KM_ProjectInfo and map a DB table named KM_PROJECT.
2) The second entity class is named KM_ProjectInfoStatus and map a DB table named KM_PROJECT_INFO_STATUS.
So the second one represent a specific field of the first one (a status of the row representd by an instance of the KM_ProjectInfo class). Infact I have something like this:
1) KM_ProjectInfo class:
#Entity
#Table(name = "KM_PROJECT")
public class KM_ProjectInfo implements Serializable {
#Id
#GeneratedValue
private Long idProjectInfo;
#Column(name = "name")
private String name;
#Column(name = "technology")
private String technology;
#ManyToOne
#JoinColumn(name = "idCountry")
private KMCountry country;
#Column(name = "power")
private long power;
#Column(name = "cod")
private String cod;
#ManyToOne
#JoinColumn(name = "idProjectInfoStatus")
private KM_ProjectInfoStatus status;
// GETTERS & SETTERS
}
2) KM_ProjectInfoStatus:
#Entity
#Table(name = "KM_PROJECT_INFO_STATUS")
public class KM_ProjectInfoStatus implements Serializable {
#Id
#GeneratedValue
private Long idProjectInfoStatus;
#Column(name = "foldertech")
private Long foldertech;
#Column(name = "folderproject")
private Long folderproject;
// GETTERS & SETTERS
}
So, as you can see in the previous snippet, the KM_ProjectInfoStatuss is a field of the KM_ProjectInfo because I want that it contains the primary key of this table as foreign key.
In the logic of my application I want that at one row of the KM_PROJECT table (so at one instance of the KM_ProjectInfo entity class) is associated a single row of the KM_PROJECT_INFO_STATUS (one instance of the KM_ProjectInfoStatus entity class) because it represent a specific status for the KM_PROJECT row.
In my code I have:
#ManyToOne
#JoinColumn(name = "idProjectInfoStatus")
private KM_ProjectInfoStatus status;
but I think that is wrong because at one row of my first table it is associated a specific single row of the second table. But maybe I am missing something about how Hibernate work.
Can you help me to understand what I am missing? What it work? Why I have #ManyToOne instead #OneToOne?
Tnx

It all depends on how you want to model things. In terms of Database structure, OneToOne and ManyToOne are implemented in the same way:
One or more JoinColumns which makes a foreign key pointing to the primary key of the other table.
So both solutions correctly map to your database, but it depends if you want to allow several KM_ProjectInfo to point to the same KM_ProjectInfoStatus, or only allow a single one.
Note that, even though you would declare a OneToOne, you could still end up with multiple KM_ProjectInfo pointing to the same KM_ProjectInfoStatus if you don't manipulate Hibernate properly.
Here you did not declare the reverse relationship, but if you did, the declaration would have to be different:
In case of a OneToOne, you would have a KM_ProjectInfo member
In case of a OneToMany (reverse of ManyToOne), you would have a Collection<KM_ProjectInfo> member

From the description it seems you want to have one-to-one relationship. That is the project entity should have its very own status not shared by any other project. You could achieve this by using #OneToOne as below.
#Entity
#Table(name = "KM_PROJECT")
public class KM_ProjectInfo implements Serializable {
#Id
#GeneratedValue
private Long idProjectInfo;
#OneToOne
#JoinColumn(name = "idProjectInfoStatus")
private KM_ProjectInfoStatus status;
}
#Entity
#Table(name = "KM_PROJECT_INFO_STATUS")
public class KM_ProjectInfoStatus implements Serializable {
#Id
#GeneratedValue
private Long idProjectInfoStatus;
#OneToOne(mappedBy="idProjectInfoStatus")
private KM_ProjectInfo project;
}
This way you can have specific status for the KM_PROJECT.
Coming back to #ManyToOne, you will want to have this if you want to share the same status with multiple projects, but that's not what you want in your case. I have tried to explain mappings in simple way here One-to-One mapping.