JPA ManyToMany join table query - java

Assuming theses Entities
#Entity
public class EntityNote implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#SequenceGenerator(name="SeqEntityNote", sequenceName="SeqEntityNote", allocationSize = 1)
#GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SeqEntityNote")
private long id;
private Date date;
private String subject;
private String content;
#ManyToMany
private List<EntityTopic> listEntityTopic;
//setters/getters
#Entity
public class EntityTopic implements Serializable {
#Id
#SequenceGenerator(name="SeqEntityTopic", sequenceName="SeqEntityTopic", allocationSize = 1)
#GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SeqEntityTopic")
private long id;
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
In my DB, a join table named "entity_note_list_entity_topic" records the ManyToMany relation.
This works correctly so far.
But I'd like to perform a count query like 'how many EntityNotes per EntitityTopic'
Unfortunatly I'm quite lost in this situation.
How this query can be written ?
Do I need other elements in my two entities ?
(In many examples I see a reverse relation using mappedBy attribute on ManyToMany.. Do I need this ?)


It will be the easiest if you make the many to many relation bidirectional. There are no serious extra costs involved, as it uses the same db structure, and the list are lazy loaded so if the relation is not being used the lists are not populated (you can hide the second direction by making accessors private).
Simply change:
#Entity
public class EntityTopic implements Serializable {
...
#ManyToMany(mappedBy="listEntityTopic")
private List<EntityNote> notes;
}
You can issue normal count jpql queries, for example:
SELECT count(n) from EntityTopic t INNER JOIN t.notes n where t.name =:name
so you don't neet to retrieve the notes and topics if don't need to.
But I also believe that your original mapping can also be queries with:
SELECT COUNT(n) FROM EntityNote n INNER JOIN n.listEntityTopic t WHERE t.name = :name


If you have the following code:
#Entity
public class EntityNote implements Serializable {
#ManyToMany(fetch = FetchType.LAZY)
private List<EntityTopic> topics;
}
#Entity
public class EntityTopic implements Serializable {
#ManyToMany(fetch = FetchType.LAZY)
private List<EntityNote> notes;
}
Then, topic.getNotes().size() will give you the number of notes associated with a topic. When using Hibernate as the JPA provider, a SELECT COUNT(...) query is issued for this instead of loading all the associated notes. If this does not work for you out-of-the-box, mark the collections as extra lazy using the instructions in this post.

Related

Spring Data/Hibernate Generates two queries instead of a JOIN

Context: I have two tables: Questionnaire and Question Section. A Questionnaire can have many Question Sections. Questionnaires and Question Sections both have Start and End Dates to determine if they are active records.
Here are my entities as written:
#Entity
#Data
public class Questionnaire {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private UUID id;
private String name;
private Date startDate;
private Date endDate;
private String description;
#OneToMany(cascade = CascadeType.All,
fetch = FetchType.LAZY,
mappedBy = "questionnaire")
#JsonManagedReference
private List<QuestionSection> questionSections = new ArrayList<QuestionSection>();
}
#Entity
#Data
public class QuestionSection {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private UUID id;
private String name;
private String description;
private int sectionLevel;
private Date startDate;
private Date endDate;
#ManyToOne(fetch = FetchType.LAZY, optional = false)
#JoinColumn(name = "QUESTIONNAIRE_ID", nullable = false)
#JsonBackReference
private Questionnaire questionnaire;
}
Here is my Spring Data Repository with a single declared method:
public interface QuestionnaireRepository extends JpaRepository<Questionnaire, UUID> {
Questionnaire findByNameAndEndDateIsNull(String name);
// Previous goal query, but worked all the way back to the above simple query
// Questionnaire findByIdAndQuestionSectionsEndDateIsNull(UUID id);
}
The above derived query generates two queries shown below:
-- For brevity
select questionnaire.id as id
questionnaire.description as description
questionnaire.end_date as end_date
questionnaire.start_date as start_date
from questionnaire
where questionnaire.name='Foo' and (questionnaire.end_date is null)
select questionsection.questionnaire_id as questionnaire id
...rest of fields here...
from question_section
where questionsection.questionnaire_id = id from above query
Then Spring Data or Hibernate is combining those two above queries into one data object representative of the questionnaire object and returning that.
My problem with this is that I would have expected One query to run with a Join between the two tables, not two and then combine the results in memory. I'm pretty experienced with Spring Data and ORMs in general and have not been able to find any documentation as to why this is happening. Honestly I wouldn't care except that my original intention was to query at the parent entity and 'filter' out children that have end dates (not active). This derived query (commented out above) exhibited the same behavior which ultimately resulted in the data set that was returned containing the end dated question sections.
I know there's 100 other ways I could solve this problem (which is fine) so this is more of an educational interest for me at this point if anyone has any insight into this behavior. I could be missing something really simple.

You should be able to do this using the Entity Graph feature introduced in JPA 2.1.
https://www.baeldung.com/jpa-entity-graph
Spring Data offers support for Entity Graphs via the #NamedEntityGraph and #EntityGraph annotations:
https://www.baeldung.com/spring-data-jpa-named-entity-graphs
So in your code:
Entity:
#Entity
#NamedEntityGraph(name = "Questionnaire.questionSections",
attributeNodes = #NamedAttributeNode("questionSections ")
)
public class Questionnaire{
//...
}
Repository:
public interface QuestionnaireRepository extends JpaRepository<Questionnaire, UUID> {
#NamedEntityGraph("Questionnaire.questionSections")
Questionnaire findByNameAndEndDateIsNull(String name);
}

public interface QuestionnaireRepository extends JpaRepository<Questionnaire, UUID> {
#EntityGraph(attributePaths = { "questionSections" })
Questionnaire findByNameAndEndDateIsNull(String name);
}

HQL strategy to equal embedded entities

i'm curious about how HQL would assert equality between an entity instances.
Let's say I have a Entity called Person
#Entity
public class Person{
#Id
private Long id;
private String name;
}
and Department
#Entity
public class Department {
#Id
private Long id;
#ManyToOne
private Person person;
}
then it's fine if I do the following statement:
Query query = getSession().createQuery("from Department d where d.person = ?");
query.setProperty(0,new Person(1L));
but, what if I have an Embedded entity and no pk defined? like
#Embeddable
public class Adress {
private String email;
private String street;
private Long identifier;
}
#Entity
public class Person{
#Id
private Long id;
private String name;
#Embedded
private Address address;
}
would have any way so I could tell JPA to make it work:
Query query = getSession().createQuery("from Person p where p.address = ?");
query.setProperty(0,new Address(1L));
even though it's not exactly a primary key?
For sure i know i'd work if I tried p.adress.identifier, and then passed just the Long value, but the point is, can I tell JPA provider how it's gonna kind of 'implement' equality my way?
Thank you all

No, it is not supported and it would be difficult in general or would not make sense in some situations, like when there are collections in the Embeddable.
If you find that you need this often though, consider converting such Embeddables to custom user types. Then you can perform comparisons the way you described.

How to make Hibernate criteria to bring only needed fields

I have next classes:
#Entity
#Table(name="A")
public class A implements Serializable{
#Id
#Column(name="a_id")
private long aId;
#ManyToOne
#JoinColumn(name="b_id")
private B b;
}
#Entity
#Table(name="b")
public class B implements Serializable{
#Id
#Column(name="b_id")
private long bId;
#Column(name="b_name")
private String name;
#Column(name="b_age")
private String age;
#OneToMany(mappedBy="b")
private Set<A> a;
}
I have getters and setters for this classes.
when I try execute next Criteria
session.createCriteria(A.class, "a_table")
.createAlias("a_table.b", "b_table")
.add(Restrictions.eq("b_table.age", "11"))
.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
.list();
In SQL generated by hibernate I have all fields from B (I mean field "name" too).
How I can make to Hibernate bring only needed fields.
In case that in criteria exists many aliases to many tables, it can increase execution time.
Thank you.

Try using projections. Maybe something like this:
Criteria cr = session.createCriteria(User.class)
.setProjection(Projections.projectionList()
.add(Projections.property("id"), "id")
.add(Projections.property("Name"), "Name"))
.setResultTransformer(Transformers.aliasToBean(User.class));
List list = cr.list();

Hibernate and #JoinFormula: org.hibernate.mapping.Formula cannot be cast to org.hibernate.mapping.Column

I'm trying to write a hibernate adapter for an old database schema. This schema does not have a dedicated id column, but uses about three other columns to join data.
On some tables, I need to use coalesce. This is what I came up with so far:
About the definition:
A car can have elements, assigned by the car's user or by the car's group of users.
If FORIGN_ELEMENT holds a user's name, definition will be 'u'
If FORIGN_ELEMENT holds a group's name, definition will be 'g'
This also means, one table (CAR_TO_ELEMENT) is misused to map cars to elements and cargroups to elements. I defined a superclass CarElement and subclasses CarUserElement and CarGroupElement.
state is either "active" or an uninteresting string
I set definitition and state elsewhere, we do not need to worry about this.
Use DEP_NR on the join table. If it's zero, use USR_DEP_NR. I did this with COALESCE(NULLIF()) successfully in native SQL and want to achieve the same in Hibernate with Pojos.
Okay, here we go with the code:
#Entity
#Table(name="CAR")
public class Car extends TableEntry implements Serializable {
#Id
#Column(name="DEP_NR")
private int depnr;
#Id
#Column(name="USER_NAME")
#Type(type="TrimmedString")
private String username;
#ManyToOne(fetch = FetchType.EAGER, targetEntity=CarGroup.class)
#JoinColumns(value={
#JoinColumn(name="GROUP_NAME"),
#JoinColumn(name="DEP_NR"),
#JoinColumn(name="state"),
})
private CarGroup group;
#OneToMany(fetch=FetchType.EAGER, targetEntity=CarUserElement.class, mappedBy="car")
private Set<CarUserElement> elements;
}
#Entity
#Table(name="CAR_GROUP")
public class CarGroup extends TableEntry implements Serializable {
#Id
#Column(name="DEP_NR")
private int depnr;
#Id
#Column(name="GROUP_NAME")
#Type(type="TrimmedString")
private String group;
#ManyToOne(fetch = FetchType.EAGER, targetEntity=Car.class)
#JoinColumns(value={
#JoinColumn(name="GROUP_NAME"),
#JoinColumn(name="DEP_NR"),
#JoinColumn(name="state"),
})
private Set<Car> cars;
#OneToMany(fetch=FetchType.EAGER, targetEntity=CarGroupElement.class, mappedBy="car")
private Set<CarGroupElement> elements;
}
#MappedSuperclass
public class CarElement extends TableEntry {
#Id
#ManyToOne(fetch = FetchType.EAGER, targetEntity=Element.class)
#JoinColumns(value={
#JoinColumn(name="ELEMENT_NAME"),
#JoinColumn(name="state"),
})
private Element element;
}
#Entity
#Table(name="CAR_TO_ELEMENT")
public class CarUserElement extends CarElement {
#Id
#Column(name="DEFINITION")
private char definition;
#Id
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumnsOrFormulas(value = {
#JoinColumnOrFormula(formula=#JoinFormula(value="COALESCE(NULLIF(DEP_NR, 0), USR_DEP_NR)", referencedColumnName="DEP_NR")),
#JoinColumnOrFormula(column=#JoinColumn(name="FORIGN_ELEMENT", referencedColumnName="USER_NAME")),
#JoinColumnOrFormula(column=#JoinColumn(name="STATE", referencedColumnName="STATE"))
})
private Car car;
}
#Entity
#Table(name="CAR_TO_ELEMENT")
public class CarGroupElement extends CarElement {
#Id
#Column(name="DEFINITION")
private char definition;
#Id
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumnsOrFormulas(value = {
#JoinColumnOrFormula(formula=#JoinFormula(value="COALESCE(NULLIF(DEP_NR, 0), USR_DEP_NR)", referencedColumnName="DEP_NR")),
#JoinColumnOrFormula(column=#JoinColumn(name="FORIGN_ELEMENT", referencedColumnName="GROUP_NAME")),
#JoinColumnOrFormula(column=#JoinColumn(name="STATE", referencedColumnName="STATE"))
})
private Car car;
}
I tried all available versions of hibernate (from 3.5.1 [first version with #JoinColumnsOrFormulas] up to 4.x.x), but I always get this error:
Exception in thread "main" java.lang.ClassCastException: org.hibernate.mapping.Formula cannot be cast to org.hibernate.mapping.Column
at org.hibernate.cfg.annotations.TableBinder.bindFk(TableBinder.java:351)
at org.hibernate.cfg.annotations.CollectionBinder.bindCollectionSecondPass(CollectionBinder.java:1338)
at org.hibernate.cfg.annotations.CollectionBinder.bindOneToManySecondPass(CollectionBinder.java:791)
at org.hibernate.cfg.annotations.CollectionBinder.bindStarToManySecondPass(CollectionBinder.java:719)
at org.hibernate.cfg.annotations.CollectionBinder$1.secondPass(CollectionBinder.java:668)
at org.hibernate.cfg.CollectionSecondPass.doSecondPass(CollectionSecondPass.java:66)
at org.hibernate.cfg.Configuration.originalSecondPassCompile(Configuration.java:1597)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1355)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1737)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1788)
Other hibernate users seem to have the same problem: They can't get it working with any version, see this thread and other stackoverflow questions:
https://forum.hibernate.org/viewtopic.php?f=1&t=1010559
To be more complete, here's my TrimmedString Class:
https://forum.hibernate.org/viewtopic.php?p=2191674&sid=049b85950db50a8bd145f9dac49a5f6e#p2191674
Thanks in advance!
PS: It works with joining just these three colulmns with just one DEP-NR-Column (i.e. either DEP_NR OR USR_DEP_NR using just #JoinColumns). But I need this coalesce(nullif()).

I ran into a similar problem, and it seems that the issue is that you are using a #Formula inside an #Id. Hibernate wants Ids to be insertable, and Formulas are read-only.
In my case I was able to work around the problem by making the individual columns Id properties on their own, and making the joined object a separate property. I don't know if this would work in your case since you're using two different columns in your formula, but if so your code might look something like:
#Entity
#Table(name="CAR_TO_ELEMENT")
public class CarUserElement extends CarElement {
#Id
#Column(name="DEFINITION")
private char definition;
#Id
#Column(name="DEP_NR")
private Integer depNr;
#Id
#Column(name="USR_DEP_NR")
private Integer usrDepNr;
#Id
#Column(name="FORIGN_ELEMENT")
private String userName;
#Id
#Column(name="STATE")
private String state;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumnsOrFormulas(value = {
#JoinColumnOrFormula(formula=#JoinFormula(value="COALESCE(NULLIF(DEP_NR, 0), USR_DEP_NR)", referencedColumnName="DEP_NR")),
#JoinColumnOrFormula(column=#JoinColumn(name="FORIGN_ELEMENT", referencedColumnName="USER_NAME", insertable = false, updatable = false)),
#JoinColumnOrFormula(column=#JoinColumn(name="STATE", referencedColumnName="STATE", insertable = false, updatable = false))
})
private Car car;
}

Join formulas are very fragile in Hibernate for the time being; I always had a difficult time to get them work properly.
The workaround that helped me often was to create database views which exposed the proper columns (including foreign keys that don't exist in the original tables). Then I mapped the entities to the views using classing Hibernate/JPA mappings.
Sometimes there are redundant joins in the generated SQL when using such entities, but the database optimizes such queries in most cases so that the execution plan is optimal anyway.
Another approach could be using #Subselects, which are some kind of Hibernate views, but I expect them to be less performant than the classic database views.

I ran into the cast exception as well and I'm on Hibernate 5.x.
Until Hibernate dedicates time to fix the issue, I found that while this guy's approach may not be cleanest (he even eludes to that fact!), it works.
You just need to add the #Column mappings (and get/set methods) to your association table objects that are returning null and manually set the values when you populate the relation data. Simple but effective!

How to get child table rows using parent table ID?

I have two tables: Organization(Parent) and Department(Child).
There is One to Many relationship, and is mentioned in Organization table only.
#Entity
#Table(name="TBL_STD_ORGANIZATION")
public class Organization implements Serializable {
#Id
#GeneratedValue
#Column(name="FLD_ORG_ID")
private Long organizationId;
#Column(name="FLD_ORG_NAME")
private String orgName;
#OneToMany(cascade=CascadeType.ALL,fetch=FetchType.EAGER)
private java.util.List<Department> listOfDepartMents = new java.util.ArrayList<Department>();
}
Below is Department Class:
#Entity
#Table(name="TBL_STD_DEPARTMENT")
public class Department implements Serializable {
#Id
#GeneratedValue
#Column(name = "FLD_DEPARTMENT_ID")
private Long departmentId;
#Column(name = "FLD_DEPARTMENT_NAME")
private String departmentName;
}
I wrote relationship in Parent table, because of it hibernate creates third table.
Now, I have to retrieve departments start with "sa" keyword and in specific organization.
So I want the HQL or SQL query query. I am not getting it how to write such complex query.
Any suggestions?

I'm fairly certain the HQL/JPQL would be:
SELECT d FROM Organization o JOIN o.listOfDepartMents d WHERE d.departmentName LIKE "sa%"

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