Hello there is my model
#Entity
public class Group {
#Id
#GeneratedValue
private Long id;
#OneToMany(
cascade = CascadeType.ALL
)
private List<User> users;
//Constructors, getters and setters removed for brevity
}
#Entity
public class User{
#Id
#GeneratedValue
private Long id;
private boolean valid;
//Constructors, getters and setters removed for brevity
}
Is it possible to make a HSQL request to select a Group by id, but keeping in the User list, only the users with 'true' in the "valid" field ?
If you want to use HSQL to return JPA entity tree (simple way), the answer is no.
Though, you can write this:
select g from Group g
left join g.users u with u.valid = true
where g.id = 123
your users collection won't be initialized. If you add fetch keyword to the join, you get an error:
QuerySyntaxException: with-clause not allowed on fetched associations; use filters
Which leads you to other solution, using annotation #Filter. But you need to enable the filter, and then you have problem with updating the entity read with the filter turned on (collection synchronization).
What is the difference between:
#Entity
public class Company {
#OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY)
#JoinColumn(name = "companyIdRef", referencedColumnName = "companyId")
private List<Branch> branches;
...
}
and
#Entity
public class Company {
#OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY,
mappedBy = "companyIdRef")
private List<Branch> branches;
...
}
The annotation #JoinColumn indicates that this entity is the owner of the relationship (that is: the corresponding table has a column with a foreign key to the referenced table), whereas the attribute mappedBy indicates that the entity in this side is the inverse of the relationship, and the owner resides in the "other" entity. This also means that you can access the other table from the class which you've annotated with "mappedBy" (fully bidirectional relationship).
In particular, for the code in the question the correct annotations would look like this:
#Entity
public class Company {
#OneToMany(mappedBy = "company",
orphanRemoval = true,
fetch = FetchType.LAZY,
cascade = CascadeType.ALL)
private List<Branch> branches;
}
#Entity
public class Branch {
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "companyId")
private Company company;
}
#JoinColumn could be used on both sides of the relationship. The question was about using #JoinColumn on the #OneToMany side (rare case). And the point here is in physical information duplication (column name) along with not optimized SQL query that will produce some additional UPDATE statements.
According to documentation:
Since many to one are (almost) always the owner side of a bidirectional relationship in the JPA spec, the one to many association is annotated by #OneToMany(mappedBy=...)
#Entity
public class Troop {
#OneToMany(mappedBy="troop")
public Set<Soldier> getSoldiers() {
...
}
#Entity
public class Soldier {
#ManyToOne
#JoinColumn(name="troop_fk")
public Troop getTroop() {
...
}
Troop has a bidirectional one to many relationship with Soldier through the troop property. You don't have to (must not) define any physical mapping in the mappedBy side.
To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the mappedBy element and set the many to one #JoinColumn as insertable and updatable to false. This solution is not optimized and will produce some additional UPDATE statements.
#Entity
public class Troop {
#OneToMany
#JoinColumn(name="troop_fk") //we need to duplicate the physical information
public Set<Soldier> getSoldiers() {
...
}
#Entity
public class Soldier {
#ManyToOne
#JoinColumn(name="troop_fk", insertable=false, updatable=false)
public Troop getTroop() {
...
}
Unidirectional one-to-many association
If you use the #OneToMany annotation with #JoinColumn, then you have a unidirectional association, like the one between the parent Post entity and the child PostComment in the following diagram:
When using a unidirectional one-to-many association, only the parent side maps the association.
In this example, only the Post entity will define a #OneToMany association to the child PostComment entity:
#OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
#JoinColumn(name = "post_id")
private List<PostComment> comments = new ArrayList<>();
Bidirectional one-to-many association
If you use the #OneToMany with the mappedBy attribute set, you have a bidirectional association. In our case, both the Post entity has a collection of PostComment child entities, and the child PostComment entity has a reference back to the parent Post entity, as illustrated by the following diagram:
In the PostComment entity, the post entity property is mapped as follows:
#ManyToOne(fetch = FetchType.LAZY)
private Post post;
The reason we explicitly set the fetch attribute to FetchType.LAZY is because, by default, all #ManyToOne and #OneToOne associations are fetched eagerly, which can cause N+1 query issues.
In the Post entity, the comments association is mapped as follows:
#OneToMany(
mappedBy = "post",
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<PostComment> comments = new ArrayList<>();
The mappedBy attribute of the #OneToMany annotation references the post property in the child PostComment entity, and, this way, Hibernate knows that the bidirectional association is controlled by the #ManyToOne side, which is in charge of managing the Foreign Key column value this table relationship is based on.
For a bidirectional association, you also need to have two utility methods, like addChild and removeChild:
public void addComment(PostComment comment) {
comments.add(comment);
comment.setPost(this);
}
public void removeComment(PostComment comment) {
comments.remove(comment);
comment.setPost(null);
}
These two methods ensure that both sides of the bidirectional association are in sync. Without synchronizing both ends, Hibernate does not guarantee that association state changes will propagate to the database.
Which one to choose?
The unidirectional #OneToMany association does not perform very well, so you should avoid it.
You are better off using the bidirectional #OneToMany which is more efficient.
I disagree with the accepted answer here by Óscar López. That answer is inaccurate!
It is NOT #JoinColumn which indicates that this entity is the owner of the relationship. Instead, it is the #ManyToOne annotation which does this (in his example).
The relationship annotations such as #ManyToOne, #OneToMany and #ManyToMany tell JPA/Hibernate to create a mapping. By default, this is done through a seperate Join Table.
#JoinColumn
The purpose of #JoinColumn is to create a join column if one does
not already exist. If it does, then this annotation can be used to
name the join column.
MappedBy
The purpose of the MappedBy parameter is to instruct JPA: Do NOT
create another join table as the relationship is already being mapped
by the opposite entity of this relationship.
Remember: MappedBy is a property of the relationship annotations whose purpose is to generate a mechanism to relate two entities which by default they do by creating a join table. MappedBy halts that process in one direction.
The entity not using MappedBy is said to be the owner of the relationship because the mechanics of the mapping are dictated within its class through the use of one of the three mapping annotations against the foreign key field. This not only specifies the nature of the mapping but also instructs the creation of a join table. Furthermore, the option to suppress the join table also exists by applying #JoinColumn annotation over the foreign key which keeps it inside the table of the owner entity instead.
So in summary: #JoinColumn either creates a new join column or renames an existing one; whilst the MappedBy parameter works collaboratively with the relationship annotations of the other (child) class in order to create a mapping either through a join table or by creating a foreign key column in the associated table of the owner entity.
To illustrate how MapppedBy works, consider the code below. If MappedBy parameter were to be deleted, then Hibernate would actually create TWO join tables! Why? Because there is a symmetry in many-to-many relationships and Hibernate has no rationale for selecting one direction over the other.
We therefore use MappedBy to tell Hibernate, we have chosen the other entity to dictate the mapping of the relationship between the two entities.
#Entity
public class Driver {
#ManyToMany(mappedBy = "drivers")
private List<Cars> cars;
}
#Entity
public class Cars {
#ManyToMany
private List<Drivers> drivers;
}
Adding #JoinColumn(name = "driverID") in the owner class (see below), will prevent the creation of a join table and instead, create a driverID foreign key column in the Cars table to construct a mapping:
#Entity
public class Driver {
#ManyToMany(mappedBy = "drivers")
private List<Cars> cars;
}
#Entity
public class Cars {
#ManyToMany
#JoinColumn(name = "driverID")
private List<Drivers> drivers;
}
The annotation mappedBy ideally should always be used in the Parent side (Company class) of the bi directional relationship, in this case it should be in Company class pointing to the member variable 'company' of the Child class (Branch class)
The annotation #JoinColumn is used to specify a mapped column for joining an entity association, this annotation can be used in any class (Parent or Child) but it should ideally be used only in one side (either in parent class or in Child class not in both) here in this case i used it in the Child side (Branch class) of the bi directional relationship indicating the foreign key in the Branch class.
below is the working example :
parent class , Company
#Entity
public class Company {
private int companyId;
private String companyName;
private List<Branch> branches;
#Id
#GeneratedValue
#Column(name="COMPANY_ID")
public int getCompanyId() {
return companyId;
}
public void setCompanyId(int companyId) {
this.companyId = companyId;
}
#Column(name="COMPANY_NAME")
public String getCompanyName() {
return companyName;
}
public void setCompanyName(String companyName) {
this.companyName = companyName;
}
#OneToMany(fetch=FetchType.LAZY,cascade=CascadeType.ALL,mappedBy="company")
public List<Branch> getBranches() {
return branches;
}
public void setBranches(List<Branch> branches) {
this.branches = branches;
}
}
child class, Branch
#Entity
public class Branch {
private int branchId;
private String branchName;
private Company company;
#Id
#GeneratedValue
#Column(name="BRANCH_ID")
public int getBranchId() {
return branchId;
}
public void setBranchId(int branchId) {
this.branchId = branchId;
}
#Column(name="BRANCH_NAME")
public String getBranchName() {
return branchName;
}
public void setBranchName(String branchName) {
this.branchName = branchName;
}
#ManyToOne(fetch=FetchType.LAZY)
#JoinColumn(name="COMPANY_ID")
public Company getCompany() {
return company;
}
public void setCompany(Company company) {
this.company = company;
}
}
I'd just like to add that #JoinColumn does not always have to be related to the physical information location as this answer suggests. You can combine #JoinColumn with #OneToMany even if the parent table has no table data pointing to the child table.
How to define unidirectional OneToMany relationship in JPA
Unidirectional OneToMany, No Inverse ManyToOne, No Join Table
It seems to only be available in JPA 2.x+ though. It's useful for situations where you want the child class to just contain the ID of the parent, not a full on reference.
Let me make it simple.
You can use #JoinColumn on either sides irrespective of mapping.
Let's divide this into three cases.
1) Uni-directional mapping from Branch to Company.
2) Bi-direction mapping from Company to Branch.
3) Only Uni-directional mapping from Company to Branch.
So any use-case will fall under this three categories. So let me explain how to use #JoinColumn and mappedBy.
1) Uni-directional mapping from Branch to Company.
Use JoinColumn in Branch table.
2) Bi-direction mapping from Company to Branch.
Use mappedBy in Company table as describe by #Mykhaylo Adamovych's answer.
3)Uni-directional mapping from Company to Branch.
Just use #JoinColumn in Company table.
#Entity
public class Company {
#OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY)
#JoinColumn(name="courseId")
private List<Branch> branches;
...
}
This says that in based on the foreign key "courseId" mapping in branches table, get me list of all branches. NOTE: you can't fetch company from branch in this case, only uni-directional mapping exist from company to branch.
JPA is a layered API, the different levels have their own annotations. The highest level is the (1) Entity level which describes persistent classes then you have the (2) relational database level which assume the entities are mapped to a relational database and (3) the java model.
Level 1 annotations: #Entity, #Id, #OneToOne, #OneToMany, #ManyToOne, #ManyToMany.
You can introduce persistency in your application using these high level annotations alone. But then you have to create your database according to the assumptions JPA makes. These annotations specify the entity/relationship model.
Level 2 annotations: #Table, #Column, #JoinColumn, ...
Influence the mapping from entities/properties to the relational database tables/columns if you are not satisfied with JPA's defaults or if you need to map to an existing database. These annotations can be seen as implementation annotations, they specify how the mapping should be done.
In my opinion it is best to stick as much as possible to the high level annotations and then introduce the lower level annotations as needed.
To answer the questions: the #OneToMany/mappedBy is nicest because it only uses the annotations from the entity domain. The #oneToMany/#JoinColumn is also fine but it uses an implementation annotation where this is not strictly necessary.
#Entity
public class Company {
#OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
#JoinColumn(name = "company_id_ref", referencedColumnName = "company_id")
private List<Branch> branches;
...
}
That Will give below Hibernate logs
Hibernate: select nextval ('hibernate_sequence')
Hibernate: select nextval ('hibernate_sequence')
Hibernate: insert into company (name, company_id) values (?, ?)
Hibernate: insert into branch (company_id_ref, name, id) values (?, ?, ?)
Hibernate: update branch set company_id_ref=? where id=?
And
#Entity
public class Company {
#OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY,
mappedBy = "company")
private List<Branch> branches;
...
}
That will give below Hibernate logs
Hibernate: select nextval ('hibernate_sequence')
Hibernate: select nextval ('hibernate_sequence')
Hibernate: insert into company (name, company_id) values (?, ?)
Hibernate: insert into branch (company_id_ref, name, id) values (?, ?, ?)
We can clearly see that #joinColumn will cause additional update queries.
so you do not need to set parent entity explicitly to child entity,
That we have to do while using mappedBy
to save children with a parent
#Entity
public class Person {
#Id
#GeneratedValue
private int personId;
#OneToOne(cascade=CascadeType.ALL, mappedBy="person", fetch=FetchType.LAZY)
private PersonDetail personDetail;
//getters and setters
}
#Entity
public class PersonDetail {
#Id
#GeneratedValue
private int personDetailId;
#OneToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
private Person person;
//getters and setters
}
when i do
Person person1=(Person)session.get(Person.class, 1);
i see two queries being fired. one for fetching person data and another for person detail data.
As per my understanding only 1 query should have been fired that is for fetching person data not for person detail data as i have mentioned
lazy loading. Why personDetail data is getting fetched along with person data ?
Hibernate cannot proxy your own object as it does for Sets / Lists in a #ToMany relation, so Lazy loading does not work.
I think this link could be useful to understand your problem: http://justonjava.blogspot.co.uk/2010/09/lazy-one-to-one-and-one-to-many.html
Based on your comment and since the PersonDetail entity contains a foreign key column that references the Person entity, it looks like you only have 1 problem:
Entity relationships include the concept of a relationship owner (in this case PersonDetail), which means that you want to add a #JoinColumn annotation in the PersonDetail entity.
You have already correctly defined the inverse side of the relationship (the entity that is not the owner of the relationship) with the mappedBy attribute that was added to the association annotation (#OneToOne in your case) to make the relationship bi-directional, which means that the associated PersonDetail may be reached from a Person instance.
Given the relationship that is clarified in your comment, you should only have to make 1 change to your code as shown here:
#Entity
public class Person {
#Id
#GeneratedValue
private int personId;
//Retain the mappedBy attribute here:
#OneToOne(cascade=CascadeType.ALL, mappedBy="person",
fetch=FetchType.LAZY)
private PersonDetail personDetail;
//getters and setters...
}
#Entity
public class PersonDetail {
#Id
#GeneratedValue
private int personDetailId;
#OneToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
//Change: add the #JoinColumn annotation here:
#JoinColumn(name="PERSON_FK_COLUMN")
private Person person;
//getters and setters...
}
I have a ManyToMany relationship established with a join table that is exactly like the one described on the Java Persistence wiki Example of a ManyToMany relationship annotations. Using that wiki example of Employees & Projects as a reference, the code listed in the example works fine to create three tables: EMP, PROJ and the EMP_PROJ join table. What I would like to do is use that EMP_PROJ join table in a separate entity. I don't want to add additional columns to the EMP_PROJ join table. For example, suppose an administrator associates projects with an employee. That list is stored in EMP_PROJ. What I would like to do is create a separate entity called ManagerReport that will return, for an employee, the list of projects associated with that employee. The manager can then enter specific info regarding each project for the employee, like start date, end date, performance, etc.
Below are the tables and sample code pulled from the wiki page.
EMPLOYEE (table)
ID FIRSTNAME LASTNAME
1 Bob Way
2 Sarah Smith
EMP_PROJ (table)
EMP_ID PROJ_ID
1 1
1 2
2 1
PROJECT (table)
ID NAME
1 GIS
2 SIG
#Entity
public class Employee {
#Id
#Column(name="ID")
private long id;
...
#ManyToMany
#JoinTable(
name="EMP_PROJ",
joinColumns={#JoinColumn(name="EMP_ID", referencedColumnName="ID")},
inverseJoinColumns={#JoinColumn(name="PROJ_ID", referencedColumnName="ID")})
private List<Project> projects;
...
}
You will have to create a ManagerReportProject entity which maps to EMP_PROJ table with #Table annotation.
In Employee entity, instead of #ManyToMany mapping for a collection of Projects use #OneToMany mapping for collection mapping to ManagerReportProject entities.
You will still be able to get a list of employee's projects because each ManagerReportProject further points to Project. You can even create a helper getProjects() method inside Employee to get a list of projects. Method must be annotated with #Transient to mark getProjects() method as not JPA persitent (by default all properties or fields inside Entity mappings are persitent)
#Entity
public class Employee
{
#Id
#Column(name="ID")
private long id;
...
#OneToMany...
private List<ManagerReportProject> managerReportProjects;
...
/*
YOU DON'T NEED THIS ANYMORE:
#ManyToMany
#JoinTable(
name="EMP_PROJ",
joinColumns={#JoinColumn(name="EMP_ID", referencedColumnName="ID")},
inverseJoinColumns={#JoinColumn(name="PROJ_ID", referencedColumnName="ID")})
private List<Project> projects;
*/
#Transient
public List<Project> getProjects()
{
List<Project> projects = new ArrayList<Project>();
for(ManagerReportProject managerReportProject: managerReportProjects)
{
projects.add(managerReportProject.getProject());
}
return projects;
}
...
}
ManagerReportProject should point to Employee, Project and Manager entity with #ManyToOne association.
Put manager report specific columns into ManagerReportProject (start date, end date, performance, etc.).
ManagerReportProject maps to EMP_PROJ table with #Table annotation.
#Entity
#Table(name= "EMP_PROJ")
public class ManagerReportProject{
#Id
#Column(name="ID")
private long id;
//manager report columns
private Date startDate;
private Date endDate;
performance, etc.
#ManyToOne...
private Manager manager;
...
#ManyToOne...
private Employee employee;
#ManyToOne...
private Project project;
...
}
Create a Manager entity, use #OneToMany for collection mapping to ManagerReportProject entities:
#Entity
public class Manager {
#Id
#Column(name="ID")
private long id;
...
#OneToMany...
private List<ManagerReportProject> managerReportProjects;
...
}
Now you can enter specific info regarding each project for the employee, like start date, end date, performance, etc.
This is a sketch just to demonstrate an idea of how to edit existing manager report for a specific empolyee working for a specific manager:
Emyployee employee = ...
Manager manager = ...
List<ManagerReportProject> managerReportProjects= employee.getManagerReportProjects()
for(ManagerReportProject managerReportProject: managerReportProjects )
{
if(manager.equals(managerReportProject.getManager()))
{
Project project = managerReportProject.getProject();
managerReportProject.setStartDate(...);
managerReportProject.setEndDate(...);
managerReportProject.setperformance(...);
...
}
}
I am using JPA (Hibernate) with the following entity class with one one-to-many relationship.
When I add elements to the list, and then persist the Organization entity, it adds the new elements to the proyects table, but when I remove elements from the list, nothing happens when persist (or merge), and I would like these elements to be removed from the database.
I tried also orphanRemoval=true in the OneToMany annotation, but it doesn't work.
#Entity
public class Organization {
#Id
#GeneratedValue
public long internalId;
#Basic
#Column(nullable = false, length = 100)
private String name;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "organization")
private List<Proyect> proyects;
// Getters and Setters
}
You need to set Proyect.organization to null and update that entity, since this property is responsible for the database entry (Proyect is the owning side in this case ).