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).
Related
I am learning persistence with Hibernate (and JPA) on Udemy and the presenter explains an alternative to the previous associations discussed (unidirectional OneToOne, bidirectional OneToOne, Unidirectional OneToMany, bidirectional OneToMany), called the #JoinTable JPA annotation.
He mentions that sometimes only a small selection of the "Many" entities are associated with a specific kind of "One" entities and that we don't want a field on the "Many" entity to embody the association with the "One" entity since that field will remain null for most instances. He goes on to recommend the #JoinTable for this situation.
My question is, considering the effect of the #JoinTable option on the Java entities, why not just use the Unidirectional #OneToMany annotation on the "One" entity and leave the "Many" entity as-is? Which additional features would #JoinTable bring to the situation from within Java beyond what Unidirectional #OneToMany brings?
Then he is clearly wrong, #JoinTable is definitely not necessary here and will introduce overhead in terms of memory (more data to store) and time complexity (another table to join).
You're right about that you only need #JoinColumn to map one-to-one and one-to-many relationships (and mappedBy on the other side if it should be bidirectional).
P.S.: I would consider going further with this course, as it seems to be flawed even with the very basics of jpa.
in my code, I have an Employee and Task entities, related with ManyToMany relation. When creating new Employee object, I can assign him to existing tasks with empToBePersisted.getTasks().add(existingTask). However, when I persist it, the relation is persisted in databse but only seen from the Employee side. The Task sees it after restarting the app. How can I make it visible immediately after persisting?
Described behaviour is normal. You need to handle both sides when working with bidirectional associations.
Bidirectional relationships between managed entities will be persisted
based on references held by the owning side of the relationship. It is
the developer’s responsibility to keep the in-memory references held
on the owning side and those held on the inverse side consistent with
each other when they change.
In this case, you will need to call existingTask.setEmployee(empToBePersisted) manually.
You can also see this answer for more details.
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.
I know that in many-to-many bidirectional association are owner and dependent entity.
What do I know about owner and dependent for writing my application?
What differencies are between them?
According to docs:
The direction of a relationship can be either bidirectional or unidirectional. A bidirectional relationship has both an owning side and an inverse side. A unidirectional relationship has only an owning side. The owning side of a relationship determines how the Persistence runtime makes updates to the relationship in the database.
I think about about owning side as "main" side of the relation. Entity on this side defines most of the properties of the relation and it's state is used to "control" relation.
I have a many-to-many relationship were the link table has an additional field. Hence the relationship is done with 2 one-to-many relations according to below tutorial:
http://www.mkyong.com/hibernate/hibernate-many-to-many-example-join-table-extra-column-annotation/comment-page-1/#comment-122181
I have the 2 entities, a third entity which defined the link table and consist of an #Embeddable ID field.
The relationship is defined as:
#OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.compound", cascade = CascadeType.ALL)
#OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.structure", cascade = CascadeType.ALL)
were pk = the #Embeddable ID field. Insert and deleting works fine however when I call
session.merge(compound);
I get a StackOverflowError and log shows that hibernate is making tons of select statements. Note that database contains exactly 1 association, eg. 1 compound containing 2 structures. It looks like hibernate gets into a endless loop.
I've seen this solution also on http://giannigar.wordpress.com/2009/09/04/mapping-a-many-to-many-join-table-with-extra-column-using-jpa/ but how do you do updated with this?
My solution was to use FetctType.EAGER on the owning side and FetchType.Lazy on child side and in the 2 ManyToOne relations in the link table. Like this I can load from the owning side without getting LazyInitializationException and mergign working as expected.
I second SpaceTuckker's answer. I don't think Persist is the same as merge. Merge will load the object before persisting it. Persist doesn't. So when you call merge IMHO it needs to load the lazy relation. If you call persist it does not.
You might also use #ElementDependent to mark related #OneToMany relations as depending on another table. This was the way I solved the Many-To-Many relation with additional columns in the join table.