I have a (classic) many to many relationship using Hibernate and a Oracle database.
I defined my entities as follows.
Student.java
#Entity
#Table
public class Student implements Serializable {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name = "STUDENT_ID")
private Long studentId;
#ManyToMany
#JoinTable(name = "STUDENT_PROJECT", joinColumns = {
#JoinColumn(name = "STUDENT_ID") }, inverseJoinColumns = {
#JoinColumn(name = "PROJECT_ID") })
private Set<Project> projects = new HashSet<>();
Project.java
#Entity
#Table
public class Project implements Serializable {
#Id
#Column(name="PROJECT_ID")
private int projectId;
#ManyToMany(mappedBy="projects")
private Set<Student> students = new HashSet<>();
I have a STUDENT_PROJECT table in my oracle database that consists of two fields, PROJECT_ID and STUDENT_ID with a composite primary key on them.
I have a sequence on my STUDENT table to auto generate their ID.
TRIGGER STUDENT_TRIGGER
BEFORE INSERT ON STUDENT
FOR EACH ROW
BEGIN
SELECT STUDENT_SEQUENCE.NEXTVAL
INTO :STUDENT_ID
FROM dual;
END;
Now my problem is that when I try to persist my Student entity, the ID field of the STUDENT table doesn't correspond to the STUDENT_ID field of STUDENT_PROJECT table.
Somehow it persists an ID for Student and a different one in the mapping table and I can't figure out why.
This is how I manipulate my objects
Student student = new Student();
// set some fields
Set<Project> projects = new HashSet<>();
// call to a private method to set its projects
student.setProjects(projects);
studentDao.persist(student);
I had to remove the foreign key in the STUDENT_PROJECT table on the ID of STUDENT (else the constraint wouldn't let me save of course) to finally notice that is was setting differents ID's but I don't understand why.
If you need more information let me know, I tried to keep it as small as possible, thanks.
UPDATE:
I have tried to remove the trigger on the STUDENT table and changed its Java configuration to
#Id
#GeneratedValue(strategy = GenerationType.AUTO, generator = "G1")
#SequenceGenerator(name = "G1", sequenceName= "STUDENT_SEQUENCE")
#Column(name = "STUDENT_ID")
private Long studentId;
I now get:
javax.persistence.EntityExistsException: A different object with the same identifier value was already associated with the session : [fr.persistance.data.Student#3037]
It doesn't save to database at all and produces the above exception on the persist(), I guess I have a problem with my sequence as it seems it tries to persist two Student objects with the same ID (I am looping to save multiples students and their projects).
I have used the eclipse debug to inspect a few of my Student objects after the persist() call and they each have a different studentId in Java but once the loop and the transaction end, the exception occurs but it does seem like they each get a separate ID.
The problem is that the Ids of the students are generated by the database in a trigger, while Hibernate tries to create the value itself.
For an oracle database, GenerationType.AUTO will use a sequence. The sequence is accessed by Hibernate, which assigns the value (before the student is actually inserted in the database).
To make Hibernate use your sequence, add
#SequenceGenerator(name = "sequence-generator", sequenceName = "STUDENT_SEQUENCE")
and remove the trigger.
When you want to use a trigger, you need to map it as GenerationType.IDENTITY. (Not recommended though.)
How it is possible to insert value in column from table that referenced from another column? For example, I have two entities:
#Entity
class A {
#Id
#Column
Long id;
#Column
String name;
}
#Entity
class B {
#Id
#Column
Long id;
#JoinColumn
A a;
#Column
//Something #Expression("a.name")
String aName;
}
In table B I have reference to table A, also it have field aName which must be value name from A table.
Does JPA have any default annotation to do this insert instead of b.setAName(a.getName())?
I have to say your situation is little bit confusing. I feel some disturbance in the force if you say aName have to be name but there is no rules in DB itself. If A and B are entities which should share name, i recommend you to exclude name as new entity and join it to A and B entities via primay key (probably some generated one). Or you can use reference to A in your B and use name from there directly if B is some kind of "child" role in entity view of point...
I've been searching over the web to find out a solution for this. It seems nobody has the answer... I start thinking i'm in wrong way adressing the problem.
Let's see if i can explain easy.
Im developing a contract maintenance. (table: contrat_mercan). For the contract, we will select a category (table: categoria), each category has qualities (table: calidad) in relation 1 - N (relationship table categoria_calidad).
This qualities must have a value for each contract where the category is selected, so I created a table to cover this relationship: contrato_categoria_calidad.
#Entity
#Table(name = "contrato_categoria_calidad")
public class ContratoCategoriaCalidad implements Serializable{
// Constants --------------------------------------------------------
private static final long serialVersionUID = -1821053251702048097L;
// Fields -----------------------------------------------------------
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "CCC_ID")
private int id;
#Column(name = "CONTRAT_MERCAN_ID")
private int contratoId;
#Column(name = "CATEGORIA_ID")
private int categoriaId;
#Column(name = "CALIDAD_ID")
private int calidadId;
#Column(name = "VALOR")
private double valor;
.... getters/ setters
In this table I wanted to avoid having an Id, three fields are marked as FK in database and first attempts where with #JoinColumn in the three fields. But it does not worked for hibernate.
Anyway, now ContratoCategoriaCalidad is behaving okay as independent entity. But I will need to implement all maintenance, updates, deletes for each case manually... :(
What I really want, (and I think is a better practice) is a cascade when I saveOrUpdate the contract as the other entities do, but I don't find the way to make a List in contrat_mercan table.
This is working perfect for other relationships in same table:
#OneToOne
#JoinColumn(name="CONDICION")
private Condicion condicion;
#OneToMany (cascade = {CascadeType.ALL})
#JoinTable(
name="contrato_mercan_condicion",
joinColumns = #JoinColumn( name="CONTRATO_MERCAN_ID")
,inverseJoinColumns = #JoinColumn( name="CONDICION_ID")
)
private List<Condicion> condiciones;
But all my attempts to map this failed, what i want, is to have in my Java entity contrat_mercan a field like this:
private List<ContratoCategoriaCalidad> relacionContratoCategoriaCalidad;
not a real column in database, just representation of the relationship.
I found solutions to join multiple fields of the same table, here, and here, but not to make a relationship with 3 tables...
Any idea? Im doing something wrong? Maybe i must use intermediate table categoria_calidad to perform this?
Thanks!!
If you want to access a list of related ContratoCategoriaCalidad objects from Contrato entity you need to declare a relationship between those two entities using proper annotations.
In ContratoCategoriaCalidad class change field to:
#ManyToOne
#JoinColumn(name = "CONTRATO_ID")
private Contrato contrato;
In Contrato class add field:
#OneToMany(mappedBy = "contrato")
private List<ContratoCategoriaCalidad> relacionContratoCategoriaCalidad;
If you want to enable cascade updates and removals consider adding cascade = CascadeType.ALL and orphanRemoval = true attributes to #OneToMany annotation.
Hope this helps!
I've got two entities with unidirectional #OneToMany mapping:
#Entity
#Table(name = "table1")
public class A {
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
Integer pk;
String name;
#OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
#JoinColumn(name = "a_pk", nullable = false)
#Where(clause = "is_deleted = 0")
List<B> children;
}
#Entity
#Table(name = "table2")
public class B {
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
Integer pk;
String content;
#Column(name = "is_deleted",nullable=false)
#NotNull
boolean deleted = false;
}
I want to obtain a list of all B entities, which are children of A entities matching a restriction.
Criteria c = session.createCriteria(A.class)
.add(Restriction.like("name", "Test%"))
.createAlias("children","b");
???
And this is where I'm stuck: c.list() will return me a list of A objects. I don't care about A, I want B. How can I do it with Hibernate criteria/projections? If it matters, I use Hibernate 4.2.12
In this simplified case it would make sense to just fetch eagerly; in the real case there's a chain of four OneToMany unidirectional associations, and I want to get all (or better yet, SOME) children all the way down the tree knowing the root id; but I don't want to load all the intermediate stuff (it's not needed, and even if join-fetching four tables would work it's plain gross). Also, simply getting the root and following down the lazy associations is a clear example of N+1 problem escalated.
As a side question, does Hibernate criteria satisfy entity/property #Where restrictions?
You can use projection to get list of "b". Like this :
Criteria c = session.createCriteria(A.class)
.add(Restriction.like("name", "Test%"))
.createAlias("children","b").setProjection(Projections.property("b"));
after this when you try to get results using c.list() it will return a list of B.
I hope this helps!!
So, yeah, I went with using HQL in the end. Nothing special.
List<B> bList = session.createQuery(
"select b from A as a join a.children as b where a.name like 'Test%'"
).list();
I have a hierarchical data structure with a fixed depth of 4. For a better understanding, let's assume the following (just an example):
The "root" level is called countries
Each country contains an arbitrary amount of states
Each state countains an arbitrary amount of counties
Each county contains an arbitrary amount of cities
So there are always 1-N relationships between the levels.
A very important usecase (given the id of a country) is to load the whole "content" of a country at once with the smallest possible impact on the performance of the database.
In a first naive approach, I created 4 entitiy classes in Java where the entity "Country" contains a list of the type "State", the entity "State" contains a list of the type "County" and so on...
But what JPA creates afterwards are of course not 4 tables, but 7 (4 for the entities + 3 for the connection between the levels due to 1-N). I don't know if this is a good solution since there is a lot of joining going on under the hood.
I also tried to map the subtypes to their parent types (a city belongs to one county, a county belongs to one state, a state belongs to one country). This results in 4 tables, but makes it more difficult to retrieve all data at once from the application's point of view. If I'm not wrong, I would need 4 different requests instead of one.
How could I solve this problem? Is there a way to combine a simple table layout (with four tables, not seven) with easy to use entity classes (a parent type should know its children)?
If not, how would you realize this?
I'm using JPA with Hibernate and PostgreSQL.
You can avoid the 3 extra mapping tables by using the #JoinColumn annotation rather than the #JoinTable annotation that I suspect you are using.
So for example:
COUNTRY
#OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy="country")
private List<State> stateList;
STATE
#ManyToOne
#JoinColumn(name="country_id")
private Country country
The database tables would be as follows:
Country
country_id => primary key
State
state_id => primary key
country_id => foreign key
This way the mapping tables between all the 4 entities can be avoided.
You can achieve this pretty easily using JPQL:
SELECT DISTINCT country
FROM Country country
JOIN FETCH country.states states
JOIN FETCH states.counties counties
JOIN FETCH counties.cities cities
WHERE country.id = :countryId
Using fetchType = FetchType.EAGER on #OneToMany/#ManyToOne(believe that one is already EAGER by default) will achieve similar results.
It's very simple use bidirectional mapping. Go through that link
How to delete Child or Parent objects from Relationship?
Make some changes like below
Country Entity:
------
#OneToMany(mappedBy="Country ",cascade = CascadeType.ALL)
private List<States > states;
#OneToMany(mappedBy="Country ",cascade = CascadeType.ALL)
private List<Counties> counties;
#OneToMany(mappedBy="Country ",cascade = CascadeType.ALL)
private List<Cities> cities;
-------
setters & getters
States Entity:
-----
#ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
#JoinColumn(name="countryId")
private Country country ;
-----
Counties Entity:
--------
#ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
#JoinColumn(name="countryId")
private Country country ;
-------
Cities Entity:
#ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
#JoinColumn(name="countryId")
private Country country ;
---------
After compilation of all entity's do your insertion . Only 4 will create and read your data by using Country object id.
You already have the solution: four table is the way to go, with bidirectional relationships (use the mappedBy property in the not-owning side of every relationship). If the relationships are EAGER-fetched, than all entities are automatically loaded. If you want to use LAZY fetching, you could try a named query in order to load the entity with all relationships loaded:
SELECT DISTINCT c FROM Country c LEFT JOIN FETCH c.states s LEFT JOIN FETCH s.counties co...
Did you try to declare the fetch type of the relations explicitely to eager with your second approach (default is lazy, that's why you have to do four queries).
E.g.
#OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
#JoinColumn ...
private ...;
see here: http://www.concretepage.com/hibernate/fetch_hibernate_annotation
Here is how your entities will look like:(You can use EAGER Loading instead of LAZY as well if you want)
Entity: Country
#Id
private Integer id;
#OneToMany(orphanRemoval=true fetch=FetchType.LAZY)
#JoinColumn(name="COUNTRY_ID")
private List<State> stateList;
Entity: State
This table has COUNTRY_ID that is Foreign Key to Country
#Id
private Integer id;
#OneToMany(orphanRemoval=true fetch=FetchType.LAZY)
#JoinColumn(name="STATE_ID")
private List<County> countyList;
#Column(name="COUNTRY_ID")
private Integer countryId;
Entity: County
This table has STATE_ID that is Foreign Key to State
#Id
private Integer id;
#OneToMany(orphanRemoval=true fetch=FetchType.LAZY)
#JoinColumn(name="COUNTY_ID")
private List<City> cityList;
#Column(name="STATE_ID")
private Integer stateId;
Entity: City
This table has COUNTY_ID that is Foreign Key to County
#Id
private Integer id;
#Column(name="COUNTY_ID")
private Integer countyId;
Your JPQL will be:
Select o from Country o where o.id=10
This will pick The Country Entity along with all the mappings like below.
Country
Holding List of States
Each States Holding List of Counties
Each Counties Holding LIst of Cities
For a requirement like yours, I would suggest to have a tree-like structure to maintain the hierarchical location data. It is relatively easy to implement & maintain and is more scalable & extensible.
In order to implement tree you need to have 2 tables LOCATION_NODE (Location ID, Location Name, Location Type[country, state, county, city]) & LOCATION_REL (Relation ID, Parent ID, Child ID). Below is the basic implementation of the tree idea.
public class LocationRel<T> {
private LocationNode<T> root;
public LocationRel(T rootData) {
root = new LocationNode<T>();
root.data = rootData;
root.children = new ArrayList<LocationNode<T>>();
}
public static class LocationNode<T> {
private T data;
private LocationNode<T> parent;
private List<LocationNode<T>> children;
}
}
This is the basic building block for a tree. You may need to add methods for add to, removing from, traversing, and constructors. But, once implemented, you have the freedom to add any new location type, change the hierarchy, add node, delete node etc with your hierarchical data.
Think out of the box.
Shishir
If you need the performance, I would suggest to de-normalize your tables and create 4 entities with following attributes (columns):
Country: id, name
State: id, countryId, name
County: id, countryId, stateId, name
City: id, countryId, stateId, countyId, name
(mapping is obvious)
Then you will be able to build a simple SQL queries.
If you need performance, prefer named queries as they are compiled at initialization time.
E.g. select all cities by country: "SELECT id, name FROM city WHERE country_id=?"
You may even not declare a references between entities using #ManyToOne, but just declare a simple #Columns. API call will, most likely, accept IDs (countryId, stateId), so you'll be better to pass that IDs as parameters to DAO. Most likely, you have a locations tables filled in once by sql script and the data should not be modified. Create foreign keys to guarantee data integrity.
And do you really need a tree-like structure in memory? If so, create it by hand, it is not very complex.
Searching Online, I found a couple of Links on JPQL which I think might help.
Link 1
Link 2
Anyways,
JPQL is one of the best ways to achieve this, try out this Query
SELECT DISTINCT country FROM Country country JOIN FETCH country.states states JOIN FETCH states.counties counties JOIN FETCH counties.cities cities WHERE country.id = :countryId
A solution that is useful, if you have relations that point to their parent only is the following:
With records:
#Entity
public class Country
{
#Id
private Long id;
}
#Entity
public class State
{
#Id
private Long id;
#ManyToOne(optional = false)
#JoinColumn(name = "country_id", referencedColumnName = "id", nullable = false)
Country country;
}
#Entity
public class County
{
#Id
private Long id;
#ManyToOne(optional = false)
#JoinColumn(name = "state_id", referencedColumnName = "id", nullable = false)
State state;
}
#Entity
public class City
{
#Id
private Long id;
#ManyToOne(optional = false)
#JoinColumn(name = "county_id", referencedColumnName = "id", nullable = false)
County county;
}
You can get all cities of a country with:
public interface CityRepository extends JpaRepository<City, Long>
{
List<City> findByCounty(County county); // county is a direct field of City
#Query("SELECT c FROM City c WHERE c.county.state.country = ?1")
List<City> findByCountry(Country country);
}