In my data model there is an entity "location" which is recursively. Furthermore there are relations to other entities.
The corresponding JPA (Spring Data JPA) entity looks like:
#Entity
#Table(name = "location")
class Location{
#OneToMany(mappedBy = "parent", orphanRemoval = true)
#OrderBy("name ASC")
Set<Location> children = null
#ManyToOne(fetch = FetchType.EAGER)
#JoinColumn(name = "parent_id")
Location parent = null
#Column(name = "name")
String name = null
#OneToMany(mappedBy = "location", fetch = FetchType.EAGER)
Stops stops = null
...
What is the most performant way to do a read only query? I just need the information inside the entity (table location) with the complete recursive structure but no information from the related entities.
I've read the phrase DTO projection, but nothing about what to do with a recursive structure.
Reading a recursive structure is usually done by making use of what SQL calls a recuresive CTE. JPA does not support that out of the box, because not all RDBMS support it. If you know that your DBMS supports it, you can make use of the following SQL to do this:
WITH RECURSIVE nodes(id, parent_id) AS (
SELECT id, parent_id FROM location l where id = ?
UNION ALL
SELECT l.id, l.parent_id FROM nodes n JOIN location l ON n.parent_id = l.id
)
SELECT id, parent_id FROM nodes
With that you get a list of a specific and all parent location ids as well as their respective parents which is flat. You will have to bring structure into this.
List<Object[]> result = //get the result of the query
Map<Integer, LocationDto> locationMap = new HashMap<>();
result.forEach(r -> locationMap.put(result.get(0), new LocationDto(result[0], result[1])));
locationMap.values().forEach(l -> l.setParent(locaitonMap.get(l.getParentId())));
If you don't want to make use of plain SQL because of portability concerns or just because you don't want to give up on your abstraction, you can make use of Blaze-Persistence which works on top of JPA and adds support for CTEs. Your query with blaze-persistence would look like this
List<LocationCte> result = criteriaBuilderFactory.create(entityManager, LocationCte.class)
.withRecursive(LocationCte.class)
.from(Location.class, "l")
.bind("id").select("l.id")
.bind("parent").select("l.parent.id")
.where("id").eq(initialId)
.unionAll()
.from(Location.class, "l")
.innerJoinOn(LocationCte.class, "cte")
.on("cte.parent").eqExpression("l.id)
.end()
.bind("id").select("l.id")
.bind("parent").select("l.parent.id")
.end()
.from(LocationCte.class)
.getResultList();
You will also need this special entity class
#CTE
#Entity
public class LocationCte {
#Id Integer id;
Integer parent;
}
Related
I have a query I'd like to run against a table, let's call it parent where I'm grabbing all rows that match a certain criteria. In SQL:
select * from parent where status = 'COMPLETE';
I have this table and another related table (child) defined as Hibernate entities such that:
#Entity
#Table(name = "parent")
public class Parent {
//...
#OneToMany(mappedBy = "parent")
private Set<Child> children;
//...
}
#Entity
#Table(name = "child")
public class Child {
//...
#ManyToOne
#JoinColumn(name = "parent_id")
private Parent parent;
#Column(name = "key")
private String key;
//...
}
I'd like my query to ALSO pull two optional child records where key is one of two values. So, in SQL:
select *
from parent p, child ca, child cb
where p.status = 'COMPLETED'
and p.id *= ca.parent_id
and ca.key = 'FIRST_KEY'
and p.id *= cb.parent_id
and cb.key = 'SECOND_KEY';
I could do this in Hibernate by just grabbing the result from the first query and iterating over the children collection looking for the rows I want but that's terribly inefficient: one query that does two outer joins vs one query + an additional query for each looking for the children I care about.
Is there a way to replicate the outer joins in the query above in Hibernate where the objects returned will have the children collection only populated with the two (or less, since they are optional) entities I am interested in?
You don't need two outer joins. You could simply use this HQL and Hibernate will add the right children to the right parent:
List<Parent> parentList = session
.createQuery("from Parent p left join fetch p.children c" +
" where p.status = 'COMPLETE' " +
" and (c.key = 'FIRST_KEY' or c.key = 'SECOND_KEY')")
.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
.list();
for(Parent parent:parentList) {
System.out.println(parent);;
}
Hope that solves your problem.
I have a simple parent-child relationship witin the same class. I like to get all instances where no parent exists (= NULL).Means the "root" categories.
I need to use criteria query and can not use HQL or SQL. Due an issue with Relationship and "isNull" Operation it seems that the criteria query can not be "normally" be used as expected and require some trick. Anybody has any hints which I can try?
#Entity
public class Category {
#ManyToOne(fetch=FetchType.EAGER)
#JoinColumn(name = "parent_id")
private Category parent;
#OneToMany(cascade=CascadeType.ALL, mappedBy="parent", fetch=FetchType.EAGER)
#OrderBy(value="pos")
private List<Category> childs = new ArrayList<Category>();
...
}
If I use the following criteria query construct I get strange result back. I get multiple times the same category instances instead just the "root" ones.
Criteria c = session.createCriteria(Category.class, "c");
c.add(Restrictions.isNull("c.parent"));
List<Category> rootCategories = c.list();
for(Category category : rootCategories) {
logger.info(category);
}
I tried already some constructs with aliases but no luck. Left Join or inner join will not work ;(
Criteria c = session.createCriteria(Category.class, "c");
c.createAlias("c.parent", "p", JoinType.RIGHT_OUTER_JOIN);
c.add(Restrictions.isNull("p.id"));
List<Category> rootCategories = c.list();
for(Category category : rootCategories) {
logger.info(category);
}
In case I use HQL (which isunfortunately no option for my issue) it works perfectly
List<Category> rootCategories = session.createQuery("FROM Category c where c.parent is null order by c.pos").list();
Thank you for any hints in advance
In my application, I have an entity A with a list of entities B that should be fetched eagerly :
#Entity
public class A
{
...
/* #OrderBy("cValue.id ASC") */
#OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
#JoinColumn(name="A_ID", nullable=false)
private List<B> BEntries = new ArrayList<B>();
...
}
#Entity
public class B
{
...
#ManyToOne
#JoinColumn(name = "C_ID", nullable = false)
private C cValue;
...
}
In order to get the list of A, I was first doing this simple query :
CriteriaBuilder critBuilder = em.getCriteriaBuilder();
CriteriaQuery<A> critQuery = critBuilder.createQuery(A.class);
Root<A> critRoot = critQuery.from(A.class);
critQuery.select(critRoot);
But there I saw that Hibernate was doing N+1 select queries on the database, 1 on class A, and N on class B (where N is the number of tuples of A in DB).
I was very surprise that, for eager fetching, Hibernate was not directly doing a LEFT JOIN query.
So I first tried to use the annotation #Fetch(FetchMode.JOIN) of Hibernate, but it was not working as expected.
So I transformed my list query with the following additional instructions:
Join<A,B> joinAB = critRoot.join(A_.BEntries, JoinType.LEFT);
joinAB.join(B_.cValue, JoinType.LEFT);
Ok, now the resulting SQL query contains all the needed LEFT JOIN to build the full A object eagerly... but it's still doing the other N queries on B table!
I first thought it was coming from the #OrderBy annotation I put on the Bentries parameter, but even when removed, it's still doing N+1 selects instead of 1...
Any idea why it's behaving like this?... and even why it's not doing a LEFT JOIN by default on eagerly fetched collections in entities?
I have an entity which has a collection of related entities.
public class Student{
#OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
#JoinColumn(name = "COURSE_STUDENT_ID" , insertable=false,updatable=false)
private Set <Course> courses;
I want to filter students by course names and student class id. For now I have worked it out how to filter by class id but I have no idea how to filter by courseId given that Student entity has a set of courses and the tables are related. I have read some articles but no code matches the one I have already.
CriteriaBuilder criteriaBuilder = persistenceStore.createCriteriaBuilder();
CriteriaQuery<Object> criteria = criteriaBuilder.createQuery();
Root<Student> root = criteria.from(Student.class);
List<Predicate> params = new ArrayList<Predicate>();
params.add(criteriaBuilder.equal(root.get("classId"),classId));
Predicate[] predicates = new Predicate[params.size()];
params.toArray(predicates);
criteria.select(root);
criteria.where(criteriaBuilder.and(predicates));
Query query = persistenceStore.createQuery(criteria);
List<Student> resultList = query.getResultList();
First of all, there is an error in your Entity: the JoinColumn annotation applies to the entity on the inverse side of the relationship, Course in your case.
So, if Course entity has a property student, Student has a property like:
#OneToMany(cascade = CascadeType.ALL, mappedBy = "student")
private Set<Course> courses;
and in Course entity you have (here it also states that in the db the table course has a field called "student":
#JoinColumn(name = "student", referencedColumnName = "id")
#ManyToOne(optional = false)
private Student student;
Read this link for an entry-level explaination on how to map entity relationships.
Regarding the Criteria Query, since you want to retrieve a List of StudentS, you can define your CriteriaQuery in a more type safe way:
CriteriaQuery<Student> criteria = criteriaBuilder.createQuery(Student.class);
Regarding the question, you have to join the tables in this way:
SetJoin<Student, Course> courses = root.join("courses");
or, using MetaModel:
SetJoin<Student, Course> courses = root.join(Student_.courses);
(had the OneToMany property been defined as a List or a Collection, you'd have had to use the corresponding ListJoin and CollectionJoin classes).
on the courses you can apply the desired Predicate conditions (supposing that Course entity has a string property called courseName):
Predicate p = criteriaBuilder.equal(courses.get("courseName"), "name-to-look-for");
or, using Metamodel:
Predicate p = criteriaBuilder.equal(courses.get(Course_.courseName), "name-to-look-for");
Finally, in order to concatenate correctly a list of predicates, you can use (at least) two techniques:
Predicate p1 = ...;
Predicate p2 = ...;
criteria.where(criteriaBuilder.and(p1, p2));
or
List<Predicate> conditions = new ArrayList<Predicate> ();
conditions.add(p1);
conditions.add(p2);
criteria.where(conditions.toArray(new Predicate[] {}));
See also this excellent article.
type mismatch: cannot convert from cascadetype to cascadetype[]
Answer is:
import javax.persistence.CascadeType;
import this line
I jave the following mapped superclass that provides a basic implementation for a parent/child self relationship to create a parent/child list for unlimited nesting of items (i.e. Categories)
#MappedSuperclass
public abstract class ParentChildPathEntity<N extends ParentChild> implements MaterializedPath<N> {
#ManyToOne(fetch=FetchType.LAZY)
#JoinColumn(name = "parent_id")
private N parent;
#Column(name = "name", unique = true)
private String name;
#OneToMany(mappedBy = "parent", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Set<N> children = new HashSet<N>();
If I load the entire table with fetch join on the parent and children, a single select loads all the records and i can happily traverse the tree. my problem comes in when i specify to retrieve a node on the tree. i want the node and all its children in a single select. below is the hql for loading the entire table:
hql.append(String.format("tree from %s tree ", tableName));
hql.append("left join fetch tree.parent ");
hql.append("left join fetch tree.children ");
if i specify the node name, i.e.:
where tree.name = :name
then hibernate retrieves the node, but when i access the children i get the SELECT N+1 issue. I realize why this is happening, (because of the tree.name = :name) but is there a way to write the HQL so it loads the specified node and all its children?
I'm just trying to figure out a way to support a simple nested item's list where i can retrieve any parent node and its children with a single select
thanks in advance,
Have you tried using the #BatchSize annotation?
#BatchSize(size = 20)
Ex:
#OneToMany(mappedBy = ..., fetch = FetchType.LAZY)
#BatchSize(size = 20)
public SortedSet<Item> getItems() { ... }
Then, if you specify the join to children in your HQL, you should be able to avoid n+1 select. I am not sure, offhand, if there is a way to specify the batch size in the HQL statement.