I have 2 classes: Driver and Car. Cars table updated in separate process. What I need is to have property in Driver that allows me to read full car description and write only Id pointing to existing Car. Here is example:
#Entity(name = "DRIVER")
public class Driver {
... ID and other properties for Driver goes here .....
#ManyToOne(fetch=FetchType.LAZY)
#JoinColumn(name = "CAR_ID")
private Car car;
#JsonView({Views.Full.class})
public Car getCar() {
return car;
}
#JsonView({Views.Short.class})
public long getCarId() {
return car.getId();
}
public void setCarId(long carId) {
this.car = new Car (carId);
}
}
Car object is just typical JPA object with no back reference to the Driver.
So what I was trying to achieve by this is:
I can read full Car description using detailed JSON View
or I can read only Id of the Car in Short JsonView
and most important, when creating new Driver I just want to pass in JSON ID of the car.
This way I dont need to do unnesessery reads for the Car during persist but just update Id.
Im getting following error:
object references an unsaved transient instance - save the transient instance before flushing : com.Driver.car -> com.Car
I dont want to update instance of the Car in DB but rather just reference to it from Driver. Any idea how to achieve what I want?
Thank you.
UPDATE:
Forgot to mention that the ID of the Car that I pass during creation of the Driver is valid Id of the existing Car in DB.
You can do this via getReference call in EntityManager:
EntityManager em = ...;
Car car = em.getReference(Car.class, carId);
Driver driver = ...;
driver.setCar(car);
em.persist(driver);
This will not execute SELECT statement from the database.
As an answer to okutane, please see snippet:
#JoinColumn(name = "car_id", insertable = false, updatable = false)
#ManyToOne(targetEntity = Car.class, fetch = FetchType.EAGER)
private Car car;
#Column(name = "car_id")
private Long carId;
So what happens here is that when you want to do an insert/update, you only populate the carId field and perform the insert/update. Since the car field is non-insertable and non-updatable Hibernate will not complain about this and since in your database model you would only populate your car_id as a foreign key anyway this is enough at this point (and your foreign key relationship on the database will ensure your data integrity). Now when you fetch your entity the car field will be populated by Hibernate giving you the flexibility where only your parent gets fetched when it needs to.
You can work only with the car ID like this:
#JoinColumn(name = "car")
#ManyToOne(targetEntity = Car.class, fetch = FetchType.LAZY)
#NotNull(message = "Car not set")
#JsonIgnore
private Car car;
#Column(name = "car", insertable = false, updatable = false)
private Long carId;
That error message means that you have have a transient instance in your object graph that is not explicitly persisted. Short recap of the statuses an object can have in JPA:
Transient: A new object that has not yet been stored in the database (and is thus unknown to the entitymanager.) Does not have an id set.
Managed: An object that the entitymanager keeps track of. Managed objects are what you work with within the scope of a transaction, and all changes done to a managed object will automatically be stored once the transaction is commited.
Detached: A previously managed object that is still reachable after the transction commits. (A managed object outside a transaction.) Has an id set.
What the error message is telling you is that the (managed/detached) Driver-object you are working with holds a reference to a Car-object that is unknown to Hibernate (it is transient). In order to make Hibernate understand that any unsaved instances of Car being referenced from a Driver about be saved should also be saved you can call the persist-method of the EntityManager.
Alternatively, you can add a cascade on persist (I think, just from the top of my head, haven't tested it), which will execute a persist on the Car prior to persisting the Driver.
#ManyToOne(fetch=FetchType.LAZY, cascade=CascadeType.PERSIST)
#JoinColumn(name = "CAR_ID")
private Car car;
If you use the merge-method of the entitymanager to store the Driver, you should add CascadeType.MERGE instead, or both:
#ManyToOne(fetch=FetchType.LAZY, cascade={ CascadeType.PERSIST, CascadeType.MERGE })
#JoinColumn(name = "CAR_ID")
private Car car;
public void setCarId(long carId) {
this.car = new Car (carId);
}
It is actually not saved version of a car. So it is a transient object because it hasn't id. JPA demands that you should take care about relations. If entity is new (doesn't managed by context) it should be saved before it can relate with other managed/detached objects (actually the MASTER entity can maintain it's children by using cascades).
Two ways: cascades or save&retrieval from db.
Also you should avoid set entity ID by hand. If you do not want to update/persist car by it's MASTER entity, you should get the CAR from database and maintain your driver with it's instance. So, if you do that, Car will be detached from persistence context, BUT still it will have and ID and can be related with any Entity without affects.
Add optional field equal false like following
#ManyToOne(optional = false) // Telling hibernate trust me (As a trusted developer in this project) when building the query that the id provided to this entity is exists in database thus build the insert/update query right away without pre-checks
private Car car;
That way you can set just car's id as
driver.setCar(new Car(1));
and then persist driver normal
driverRepo.save(driver);
You will see that car with id 1 is assigned perfectly to driver in database
Description:
So what make this tiny optional=false makes may be this would help more https://stackoverflow.com/a/17987718
Here's the missing article that Adi Sutanto linked.
Item 11: Populating a Child-Side Parent Association Via Proxy
Executing more SQL statements than needed is always a performance penalty. It is important to strive to reduce their number as much as possible, and relying on references is one of the easy to use optimization.
Description: A Hibernate proxy can be useful when a child entity can be persisted with a reference to its parent ( #ManyToOne or #OneToOne lazy association). In such cases, fetching the parent entity from the database (execute the SELECT statement) is a performance penalty and a pointless action. Hibernate can set the underlying foreign key value for an uninitialized proxy.
Key points:
Rely on EntityManager#getReference() In Spring
use JpaRepository#getOne() Used in this example,
in Hibernate, use load()
Assume two entities, Author and Book, involved in a unidirectional #ManyToOne association (Author is the parent-side) We fetch the author via a proxy (this will not trigger a SELECT), we create a new book
we set the proxy as the author for this book and we save the book (this will trigger an INSERT in the book table)
Output sample:
The console output will reveal that only an INSERT is triggered, and no SELECT
Source code can be found here.
If you want to see the whole article put https://dzone.com/articles/50-best-performance-practices-for-hibernate-5-amp into the wayback machine. I'm not finding a live version of the article.
PS. I'm currently on a way to handle this well when using Jackson object mapper to deserialize Entities from the frontend. If you're interested in how that plays into all this leave a comment.
Use cascade in manytoone annotation
#manytoone(cascade=CascadeType.Remove)
Related
I'm newbie in Hibernate and I'm trying to learn about JPA and Hibernate.
I want to know that what is the reason that Hibernate does not allow to save the object which references an unsaved transient instance? I want to know WHY this is a problem?
I asked someone and some of them answer me like this:
How could we possibly map the customer to the address, if there is no
adress record in the DB yet?
and
you are assigning particular Address to Customer. But Address does
not have any ID
but honestly I can't understand them.
(I know that an exception will be thrown and the solution is Cascade but I want to the reason of the problem inside the database)
now, let's assume we have all of these code:
(I use Bidirectional One-To-One relationship for my example)
public class Customer {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String firstName;
private String lastName;
#OneToOne(mappedBy = "customer")
private Address address;
}
#Entity
public class Address {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String street;
private String zipCode;
#OneToOne
private Customer customer;
}
public static void main(String[] args) {
EntityManager entityManager = emf.createEntityManager();
entityManager.getTransaction().begin(); // Begin Transaction
Customer c1 = new Customer("Mi", "S");
Address addrss1 = new Address("5412 S 5th", "212524");
c1.setAddress(addrss1);
addrss1.setCustomer(c1);
entityManager.persist(c1);
entityManager.getTransaction().commit(); // Commit
entityManager.close();
}
and let's assume that the exception is not thrown and java and hibernate have allowed us to run our code and this is our customer table.
id firstName lastName
---------------------------------
1 Mi S
and this is our address table:
id street zipCode customer_id
---------------------------------------------
- - - -
now, what is the problem? everything in these Bidirectional One-To-One relationship seems right.
then what is the problem?
PS:
if it is possible, please explain and show me code.
I can understand better with code. thank you.
I want to see for example if we are allowed to save the object which references an unsaved transient instance, what problems will we face in our code and in our tables (for example do we have any problem when we want to retrieve a customer and etc)
Because your adress entity have the primay key of customer as a foreign key ,(since mappedby is in Customer entity) ,and the customer referenced by the adress has no id ,which tells hibernate that that entity was never persisted in the database (which literally means transient) ,and hibernate needs a persisted/managed entity to make sure it exists in the database so that the adress object can be associated with an existing customer.
Customer is new, and it is clear from the persist call you want to insert it, but it isn't clear what you want to happen to any of customer's references. To make it clear, you define what you want the JPA provider (Hibernate) to do in the mappings under any/all circumstances - this is what the cascade operations refer to. In this case, JPA will look at the customer.address OneToOne mapping and find nothing defined; Address is NOT managed in this EntityManager context, so it doesn't know what to do to handle this relationship, so it signals you've made a mistake by throwing an error.
If it let it through, your Customer instance references something that does not exist, and its state does not match what is in the database. What you pass into persist should be what you would get back on reads, so it should reflect the state that is in the database.
The issue isn't directly with your persist call, as the spec does allow providers to ignore references to detached/new instances that don't have cascade settings - what happens is just undefined. Where you go wrong in this situation is on flush/commit, which is when the persistence unit is synchronized to the database (section 3.2.4 of JPA 3.0), which requires providers to go through managed entities and then determine any changes. Adding a new address pre persist will result in the same issue as if you did it post persist, and requires providers to throw an IllegalStateException if it discovers new or removed entities and rollback the transaction.
Why this is a problem: JPA is very big on entity Identity, as this enables caching of these entities in multiple levels of caches, and this entity might go into those caches as it is. It has to know what to do with references to entities that do not exist, and the spec decided to require an exception. Even to your app this is and should be a problem, as the EntityManager context is a unit of work, and the state within that unit of work is based on something that is wrong. Your Customer doesn't really have an address when this is said and done, yet your application business logic thinks it assigned one, with state that just isn't going to be there afterward.
You already know the solutions:
correct the customer to have a valid, managed address by calling persist on it directly in this same EntityManager context.
set the cascade options on the mapping to cascade persist to address for you
don't set addresses on a new customer in the same operation.
I have an old database where there are two tables with implicit association between them:
booking
- id
- name
... some other fields...
and
booking_info
- id
- booking_id
... some other fields...
Due to the current database design there no any constraints between these two tables, which means that one booking entry may exist without any booking_info entries and vice versa. booking_id in booking_info table is an implicit foreign key which refers to booking table (column id), but it also may refer to the absent booking.
I have created the following JPA mapping for these tables:
#Entity
public class Booking {
#Id
private Long id;
private String name;
// getters & setters
}
and
#Entity
public class BookingInfo {
#Id
private Long id;
#OneToOne
#JoinColumn(name = "booking_id")
private Booking booking
// getters & setters
}
Now I need to be able to persist a BookingInfo entity even if there's no related Booking entry in the database.
BookingInfo bookingInfo = new BookingInfo();
bookingInfo.setId(1);
Booking booking = new Booking();
booking.setId(182); // let's say that there's no booking with id 182 in my database, but I still need to persist this bookingInfo
bookingInfo.setBooking(booking);
bookingInfoRepository.save(bookingInfo); // using Spring Data JPA
If I run this code then I get javax.persistence.EntityNotFoundException since booking with id 182 is absent.
What would be the proper workaround for my case using JPA or Hibernate.
Btw, I also tried to use Hibernate's #NotFound annotation. As a result, save method doesn't throw javax.persistence.EntityNotFoundException and entity gets persisted int the database, but the problem is that booking_id in the database always null.
Any help would be appreciated.
I am not sure my answer will help you or not, but the result you are getting perfectly make sense. As you are setting a JPA object, and that object is not present, hence the null value is saved.
If you want to save 182 as an integer, you don't do JPA relationship. Instead, you just use booking-id as an integer field in booking-info. And that makes sense because you actually do not have the relationship between those tables which the JPA is trying to achieve.
But I am sure you just want to save 182 and as well as maintain the JPA relationship. And I am sure you already know it, but DB integrity is not being maintained with the approach you are taking. I am sure there is enough reason behind that. But my recommendation would be applying proper constraints in the DB and then in JPA.
I'm coming from eclipselink and try to work myself through Hibernate.
Lets assume we have a class Car and a class Wheel. The Car class has n wheels. Both entities are connected with a bidirectional association. More importantly on the Wheel side I have a Car reference:
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "car_id")
private Car car;
plus a getter.
Now I want to fetch a wheel using its id. From my EntityManager (not a hibernate Session). I initialize the EntityManager like this:
EntityManagerFactory emf = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
EntityManager em = emf.createEntityManager();
The next step is to fetch a wheel like this:
Wheel wheel = em.find(Wheel.class, 1);
The wheel return the wanted class and its fine. Now I want to know which car is the parent of the wheel with something like:
Car car = wheel.getCar();
With eclipselink, the actual car would have been loaded. With hibernate a proxy class is loaded instead.
The only solution I worked out so far is to set the FetchType.EAGER or directly fetch join the relationship. What I realized is that the SQL statement in Hibernate is still executed, but no real object delivered. Also after
Hibernate.initalize(car)
I cannot retrieve the car entity.
Is there any way to get the expected object back without forming a query or an eager fetch?
You probably don't need to worry about the proxy. The proxy should return all of the properties in the same way that the normal object will.
If the proxy object is not working (it is returning null values), it might be that some of your fields or setters or getters are set to final. Check that out first.
You need to use the Hibernate specific LazyToOneOption.NO_PROXY annotation:
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "car_id")
#LazyToOne(LazyToOneOption.NO_PROXY)
private Car car;
This will instruct Hibernate to load the actual object, instead of giving you a Proxy:
Lazy, give back the real object loaded when a reference is requested
(Bytecode enhancement is mandatory for this option, fall back to PROXY
if the class is not enhanced) This option should be avoided unless you
can't afford the use of proxies
But you'll have to use Bytecode instrumentation to activate this option.
I'm using Spring 3.2 with Roo 1.2.3 to build a database-backed Java application via Hibernate. I have several bidirectional OneToMany/ManyToOne relationships among the tables in my database. When I set up the ManyToOne side of the relationship using #JoinColumn (via "field reference" in Roo), a new field whose type is the related entity (the "one" in ManyToOne) is created. However, once this is done, there seems to be no way to access the underlying column value on which the ManyToOne relationship is based. This is a problem when the underlying join column contains data needed by the application (i.e. when the join column contains product stock numbers).
Is there any way to set up my entity class so that the column on which its ManyToOne relationship is based remains accessible without traversing the new join property? How can I define an accessor method for the value of this column?
I've been looking online for an answer to this question for several days, but to no avail. Thanks in advance for your help.
just map the column a second time with insertable=false and updateable=false
To make it more concrete. It's possible to do a HQL-SELCT and restrict a ManyToOne relationship, without any join in the resulting SQL:
Instead of using a join in
session.createQuery("FROM Person person WHERE person.adress.id = 42")
we use can use the adress_idcolumn
session.createQuery("FROM Person person WHERE person.adressId = 42")
This works, if you specify an additional adressId field, which is only used as mapping info for Hibernate:
#Entity
#Access(AccessType.FIELD)
public class Person{
#Id
String id;
#JoinColumn(name = "adress_id")
#ManyToOne(fetch = FetchType.LAZY)
#Nullable
public Adress adress;
#Column(name = "adress_id", insertable = false, updatable = false)
private String adressId;
}
#Entity
#Access(FIELD)
public class Adress{
#Id
String id;
}
The AccessType.FIELD is not needed (But we can leave getters/setters in example). The FetchType.LAZY and #Nullable are also optional, but make it clear when it makes sense to use it. We are able to load Person entities which have a specific Address (we know the address id). But we don't need a join because it's not needed for the WHERE-clause and not for the initial fetch (the address can be fetched lazy).
I am trying to establish a relationship between 2 entities which would be zero-to-one. That is, the Parent can be saved without the associated Child entity and also along with the assoicated Child.
Following are the 2 Entity classes...
Employee (Parent)
public class Employee {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#Column(name="EMP_NAME")
private String name;
#PrimaryKeyJoinColumn
#OneToOne(cascade = {CascadeType.ALL})
private EmployeeInfo info;
#Column(name="EMP_ENUM")
private Integer enumId;
EmployeeInfo (Child)
public class EmployeeInfo {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private Long id;
#Column(name="EMPLOYEE_EMAIL")
private String email;
With such kind of a relation and id column of the only Parent (Employee) table set to AUTO INCREMENT in MySql DB, the problem is that while saving a Parent->Child object graph, I get the following exception
org.springframework.orm.hibernate3.HibernateJdbcException: JDBC exception on Hibernate data access: SQLException for SQL [insert into EMP_INFO
Caused by: java.sql.SQLException: Field 'id' doesn't have a default value
I tried setting the Child Table's Id property to AUTO INCREMENT in the DB , and the persistence of such a Parent->Child object graph is successful.
However, the problem described here surfaces, because I have a scenario in which I would like to save the parent (Employee) object without the associated EmpInfo object, and hence do NOT want to have AUTO INCREMENT on the Child's id column.
One solution could be not use the PrimaryKeyJoinColumn, but use a particular JoinColumn, but that adds an unnecessary column to my existing Table.
Has anyone come across such a problem? If yes, any pointers would be much helpful.
Finally, I got it working thanks to Pascal and some googling from my side. Apparently, I cannot use the Native key generator for such relationships where the parent can exist without the child (optional = true).
The thing that worked finally was the following, leaving me the downside of having to deal with Hibernate specific annotation (#GenericGenerator) and also having to make-do with bi-directional relationships instead of the unidirectional that I wanted.
Employee (Parent) class remains unchanged as above. It has AUTO INCREMENT on the Id column.
As for the child class (EmployeeInfo) it changed to the following, and again WITHOUT having the AUTO INCREMENT set on the Id column.
#Table(name="EMP_INFO")
#Entity
public class EmployeeInfo {
#Id
#GeneratedValue(generator="foreign")
#GenericGenerator(name="foreign", strategy = "foreign", parameters={
#Parameter(name="property", value="verifInfo")})
private Long id;
#OneToOne(optional=false)
#JoinColumn (name="id")
private Employee emp;
#Column(name="EMPLOYEE_EMAIL")
private String email;
This helped me achieve what I wanted but on the downside, GenericGenerator is not a JPA annotation, it is a hibernate annotation, and sadly I have to make do with that as of now because JPA does not currently support this(or any similar) annotation.
Anyway, it helps to get through such cases :-)
I have a scenario in which I would like to save the parent (Employee) object without the associated EmpInfo object.
The optional attribute of a OneToOne is true by default, which is what you want.
However, you are somehow misusing the #PrimaryKeyJoinColumn here (well, it actually depends on what you really want to achieve but your current combination of annotations is not correct).
IF you want to map a OneToOne with a shared primary-key, use the #PrimaryKeyJoinColumn. But in that case, don't use a GeneratedValue on EmployeeInfo and set the id manually or, if you don't want to set it manually, use the Hibernate specific foreign generator that I already mentioned in your previous question. Check also the related question mentioned below.
And IF you do not want to use a shared primary key (like in your current code since you're trying to get the id generated by the database), then do not use the PrimaryKeyJoinColumn.
You have to make a choice.
References
JPA 1.0 specification:
9.1.32 PrimaryKeyJoinColumn Annotation
Related question
JPA Hibernate One-to-One relationship.