I have the following #OneToOne relationship:
#Entity
public class CarUser {
#OneToOne
#JoinColumn(name = "use")
private User user;
}
#Entity
public class User {
}
Basically, the User is in the core model and CarUser is in an extension model. And User shouldn't know anything about CarUser (I cannot define an inverse relation on it).
The question is When I delete the User, is there anyway I can cascade delete the CarUser as well?
By definition, if you want to state that "User shouldn't know anything about CarUser" you can't get the persistence layer to cascade for you.
You don't necessarily need to make that statement, though - it may not be a correct design understanding. It's reasonable for entities, within the same database schema, to know about each other.
Let's put it this way -- even though (in an manufacturing/accounting system) CostingModule and LedgerModule are in different modules, they absolutely are expected to communicate and interact.
You can also do it with a foreign-key constraint in the database instead, or with triggers.
alter table CARUSER add constraint CARUSER_USER foreign key (ID)
references "USER" (ID) on delete cascade;
Related
After some frustrating issues and tests, I've read that hibernate can't lazily fetch ToOne relationships.
From what I've read, hibernate lazily fetches ToMany by setting its own Set as a proxy, and when a method is called on that Set, it fetched the data in the database before performing the action. Fine.
For ToOne, the reason I've seen is that since the attribute can be null (unlike ToMany), hibernate has to know whether it needs to populate it with null or a proxy, and that hibernate cannot know that without querying the other table. And since it has to query that other table, it eagerly fetches the data at the same time.
I find that rather stupid. I can understand it on the non owning side of the relationship, where nothing in the table indicates whether the toOne is populated, but on the owning side, the table contains a column with the foreign key, which is either null or not.
Why can't hibernate query the table and set the attribute to either null or a proxy depending on the value from that column? It doesn't need to check the second table, if your column is not null, you know the second table has a corresponding entry (and if it hasn't, you have an integrity problem and hibernate should just throw).
Hibernate behaves more or les how you descibed.
On the owning side hibernate supports Lazy loading, it's just not enabled by default. You need to add it #OneToOne(fetch = FetchType.LAZY)
But when you have that mapped bidirectionaly (on both entities), as you said hibernate needs to query the table to decide between null or proxy. So devs decided to eager load the whole entity. Regardless of the fetch type.
You can avoid theese problems by getting rid of the foreign key and just use same primary key vaue.
You can do that with #MapsId annotation on the owning side.
#Entity
public class Owning {
#Id
private Long id;
#OneToOne
#MapsId
#JoinColumn(name = "id")
private Child child;
}
I have a Task entity that should contain an array of User entities in a #OneToMany relationship.
But I can't figure out a way to store said array without having a link table, since a User may be referenced in multiple Tasks.
The relationship should look like such, in TaskEntity:
#OneToMany(mappedBy = "user")
private Set<UserEntity> users = new HashSet<>(0);
What would be the proper way of doing it? How can a single row save an array of entities?
You can avoid join table (but it is not advised to do so, see below) for unidirectional one-to-many relationship. It would require foreign key to be on the target side of relationship(many side of relationship). You would need to specify a #JoinColumn annotation to point to the foreign key column.
#OneToMany
#JoinColumn(name="TASK_ID")
private Set<UserEntity> users = new HashSet<>(0);
It is not advisable, because of the following :
1) Performance : if both UserEntity state and Task states are changed, upon writing to UserEntity the FK to Task is not known, because UserEntity does not have reference to it. So in this case UserEntity might be written twice, once for UserEntity changes and once for Task changes.
2) Mapping : if UserEntity is assigned to a different task, and there is no reference back to Task from UserEntity, there will be no changes in context.
So it is advisable to go for Join Table in your case.
So a User can have many Tasks and each Task can have many Users.
Sounds like a classic fan-trap, you will need a link table and to use #ManyToMany.
However the Data-Structure should be modeled before looking into data access implementation.
What you have described is a Many to Many relationship, and your database structure should reflect that, before you think about how you can retrieve the data with Hibernate.
I am confused in owner in relationship with hibernate.
what exactly is owner(owner side) in association?
I want to study Mappedby and inverse.
please help.
As a general rule the owning side of a relation would the side which you'd need to update for the change of the relation to be persisted.
If you are mapping the entities to a relational database (most probably the case) the owning side often can be identified as the entity whose table contains the foreign key.
In the entities themselves mappedBy would refer to the owning side and thus is placed on the inverse side of the relation.
In 1:n relations in most cases the owning side is the n side, in n:m relations, 1:1 relations or 1:n with mapping tables you can choose either side, just choose one.
Example:
class Thread {
#OneToMany( mappedBy = "thread" )
List<Entry> entries;
}
class Entry {
#ManyToOne
Thread thread;
}
In the example, the owning side would be the Entry entity, since you need to change the value of Entry#thread to change the thread an entry belongs to. Just adding/removing an entry to/from Thread#entries wouldn't make the changes persisted in most cases (orphanRemoval and the like would still have an effect if done correctly).
Recently, I have been learning about Hibernate, and I am facing some difficulties. My first problem is as follows: I am very much confused with the below terms.
Bidirectional mapping
Many to One
Because, as far as I know, in rdbms we first need to insert in parent table. Then we can insert on child table, so the only possible scenario is one-to-many (first parent then children). Then, how is many-to-one is going to work? Second, what is this bidirectional mapping in regards to Hibernate. Specifically, different types of join annotations confuse me a lot. I am listing those annotations below.
1.#JoinTable(name = "Tbale_Name", joinColumns = { #JoinColumn(name = "Column_Name") },
inverseJoinColumns = { #JoinColumn(name = "Another_ColumnName") })
2.#OneToMany(mappedBy="department")` this mappedby term
3.#PrimaryKeyJoinColumn
Please help me understand these concepts.
The first thing I would say is don't think in terms of tables but think in terms of Objects.
What you are trying to express with the annotations is the relationship between objects, let hibernate work out how to persist the data. You can obviously manually check the SQL but the idea of using an ORM is to map the relationships between entities accordingly and let the ORM figure out the complexity around generating SQL etc.
Its worth noting that the parent -> child relation can be mapped using #ManyToOne by adding mappedBy to the non-owning (child) side of the relationship. Hibernate then will determine which entities to insert into the database first. Running with a TransactionManager will enforce integrity with multi table inserts. Hibernate will also workout which entities need to be persisted, for example if you add an new object on the many side to an existing object on the one side.
Furthermore, its worth understanding that in some cases it won't always be the database that generates the primary key in a parent -> child foreign key. Its possible for the code to generate the Identifier and hibernate will persist them according.
Bidirectional mapping means that object entities have a reference to each other. i.e. You can retrieve the second entity from the first entity. Bidirectional mapping supports one-to-many
or many-to-many. I.e. OneToMany = a Set on one of the entities. Many-To-Many = Sets on both entities.
JoinTable tells hibernate that a table in the database can be used to map to other tables together. See JPA "#JoinTable" annotation for more information. JoinColumn tells hibernate what column to use to make the join between the two entities. Hibernate needs these to construct the SQL.
My current project uses JPA and HSQLDB.
I would like to persist multiple related objcts at one go, is that by any means possible in JPA?
Ex: Suppose there are two entities like Person and ContactInfo, where Person has List<ContactInfo> entities.
If I want to persist Person entity along with ContactInfos also, what I am doing is set the list in Person and call persist. Will doing that take care of persisting List<ContactInfo> also? (With foreign key reference to Person ID in database table)
Else kindly let mek now how would I achieve this in JPA.
Regards,
Satya
It will, if you set #*ToMany(cascade=CascadeType.PERSIST)
You could do as Bozho suggested, but if you would also like them to be updated, deleted, etc. when it's done with Person, I would suggest to cascade like that:
#OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
Note: orphanRemoval will only work with JPA 2.