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.
Related
here I'am, trying to get a better understanding of JPA Relationships in SpringBoot.
Those Entities has been created following some XML configuration files.
I'm not used to XML and I'm trying to improve my Java/SpringBoot #Annotation skills.
I'll show you 3 Entities with a OneToOne & ManyToOne annotations.
Questions
In Driver -< ServiceRequestDriverVehicle (#OneToMany) the mappedBy="driverId" references the current Driver id field or the JoinColumn(name="driverId") from the other Entity? e.g.: In Vehicle Entity mappedBy use vehicle, which if comes from ServiceRequestDriverVehicle it's named as vehicleId #JoinColumn(name="vehicleId") but it's field name is vehicle, so why where is using vehicle instead of vehicleId as in the Driver case does? Again this is abstracted from an XML configuration that can be wrong.
Are the arrows from the images pointing the right meaning of the Annotation? I mean, is the annotation taking the entity/class of the annotated field to make the relationship mapping?
In the #OneToOne relationship(Driver), is the referencedColumnName="ID" mandatory/necessary or OneToOne is automatically linked to the PK (id)
I have a MySQL model where the Set fields do not appear, should I Annotate those as #Transient or the LAZY fetch is enough?
Thank you for your answers, I hope this can solve future misunderstandings
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.
I have a database that, there is relations between some of tables of it,
for example, I have to table Person and Company, and there is a one to many relation between their Id that is build in a table name Person_Company generated by hibernate.
when I use #OnetoMany and #ManytoOne annotations in java,
It make relation between them, but not according to Person_Company table
and I use just Id 's of table
what is correct annotation?
You want the association to be mapped using a join table. So the annotation is (drumroll...) #JoinTable. The javadoc has examples, and the Hibernate documentation as well.
This question is related to this in relation to the fact that I have a one-way #ManyToOne relationship between child and parent. There are hundreds of thousands of child entries, and I do not want the parent to have a #OneToMany relationship with the children.
All works fine except for the fact that deleting the parent is not possible unless I delete the children first, and I am looking to achieve what Postgres easily provides through the ON DELETE CASCADE. I am using Postgres 9. I am also using JPA 2.0/Hibernate 4.1.7.
I am currently using the auto DDL creation feature of Hibernate, because I am frequently moving things around, so I don't want to manually add the ON DELETE CASCADE on the tables, because they will be deleted the next time round I do some changes.
Is there any way I can put the ON DELETE CASCADE constraint/trigger as part of the JPA #ManyToOne annotations in the child Entity? This way the database is created correctly automatically.
Using JPA annotations, no. But Hibernate has #org.hibernate.annotations.OnDelete.
#ManyToOne
#JoinColumn
#OnDelete( action = OnDeleteAction.CASCADE )
private Parent parent;
The EG is discussing adding something similar this as part of JPA 2.1
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.