Selecting nested Hibernate immutable objects throws org.hibernate.hql.internal.ast.QuerySyntaxException: unexpected token: new

Question

Using:

Hibernate 4.1.6

Spring 4.1

Java 8

I have two immutable data models:

Parent model

@Entity

@Immutable

@Table(name="PARENT_TABLE")

public class Parent {

    @Column(name="NAME")

    private final String name;



    @OneToMany(cascade=CascadeType.ALL, mappedBy="parent")

    @MapKey(name="key")

    Map<String, Child> children = new HashMap<>(1);



    public Parent(String name) {

        this.name = name;

    }



    public putChild(Child c) {

        Child childWithRef = new Child(this, c.getKey());

        children.put(c.getKey(), childWithRef);

    }

}

Child model

@Entity

@Immutable

@Table(name="CHILD_TABLE")

public class Child {

    @ManyToOne

    @JoinColumn(name="PARENT_ID") //say Parent has a generated ID

    private final Parent parent;



    @Column(name="KEY")

    private final String key;



    public Child(Parent p, String key) {

        this.parent = p;

        this.key = key;

    }

}

And I want to fetch all the children, with their parents, so I try running (

entityManager

is from Spring):

entityManager.createQuery("SELECT new Child(new Parent('sample name'), c.key) FROM Child c").getResultList()

But I get an exception (the problem indicated is with the second

new

):

org.hibernate.hql.internal.ast.QuerySyntaxException: unexpected token: new near line 1, column ...

Does anyone know how to fetch nested immutable objects?

I know Hibernate is really against immutability. Since I'm only performing simple database tasks, I'd like to try nonetheless.

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Answer 1

Your query is illegal for JPQL. The JPQL BNF (JPA 2.1 spec) states very clearly

select_expression ::= single_valued_path_expression | scalar_expression | aggregate_expression | 
    identification_variable | OBJECT(identification_variable) | constructor_expression
constructor_expression ::= NEW constructor_name ( constructor_item {, constructor_item}* )
constructor_item ::= single_valued_path_expression | scalar_expression | aggregate_expression |
    identification_variable

You cannot nest constructor expressions.

Extract it out into a single object and then split into your structure in your code. For example

SELECT c.key FROM Child c

to get the "key" of the child, and then create the objects yourself in your code.

EDIT FOR FULL CODE:

Here is the full solution in case someone needs it too (there are probably many ways to improve and add performance, so use this as base for understanding):

//fetch all fields without instantiating any models from Hibernate;
//all aliases are for understanding only;
//notice `INNER JOIN c.parent` for fetching the parent data
List<Object[]> results = entityManager.createQuery("SELECT p.id as parentId, p.name as parentName, p.key as parentKey, c.key as childKey FROM Child c INNER JOIN c.parent").getResultList()

//use map for parents identification
Map<Long, Parent> parentsMap = new HashMap<>(results.size())

//now use the data to instantiate the immutable models
results.forEach(resultRecord -> {
    Long parentId = (Long) resultRecord[0];
    if (parentsMap.get(parentId) == null) { //first time meeting this parent
        Parent p = new Parent(resultRecord[0], resultRecord[1], resultRecord[2]);
        parentsMap.put(parentId, p);
    }

    Child c = new Child(parentsMap.get(parentId), resultRecord[3]);
    parentsMap.get(parentId).putChild(c);
}

Selecting nested Hibernate immutable objects throws org.hibernate.hql.internal.ast.QuerySyntaxException: unexpected token: new

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