How do I map entity association using a query instead of a @JoinColumn with JPA?

Question

I’m using Hibernate 4.1.3.Final with JPA 2.1 and MySQL 5.5.37. I have an entity with the following field:

@Entity
@Table(name = "category",
       uniqueConstraints = { @UniqueConstraint(columnNames = { "NAME" })}
)
public class Category implements Serializable, Comparable<Category> {
    private static final long serialVersionUID = 1L;

    @Id
    @Column(name = "ID")
    @GeneratedValue(generator = "uuid-strategy")
    private String id;

    @NotEmpty
    private Set<Subject> subjects;

    ...
}

There is no simple join table to link up the subjects field, instead, there is a slightly more complex MySQL query. Here is an example of figuring out the subjects given a particular category id:

SELECT DISTINCT e.subject_id 
FROM category c, resource_category rc, product_resource pr, 
     sb_product p, product_book pe, book e 
WHERE c.id = rc.category_id 
  AND rc.resource_id = pr.resource_id 
  AND pr.product_id = p.id 
  AND p.id = pe.product_id 
  AND pe.ebook_id = e.id 
  AND c.id = ‘ABCEEFGH‘;

What is the simplest way to wire up the above field using the query below when loading categories?

This question concerns dealing with Java to accomplish this so building a view or doing some other type of MySQL madness is not an option, at least as an answer for this question.

Edit:

Added the notation per the suggestion (replacing '="ABCDEFG"' with '=id') but Hibernate is generating this invalid SQL when I do queries for items tied to the Category entity. Here is the SQL Hibernate spits out

SELECT categories0_.resource_id AS RESOURCE1_75_0_,
       categories0_.category_id AS CATEGORY2_76_0_,
       category1_.id            AS ID1_29_1_,
       category1_.NAME          AS name2_29_1_,SELECT DISTINCT e.subject_id
FROM            category c,
                resource_category rc,
                product_resource pr,
                product p,
                product_ebook pe,
                book e
WHERE           c.id = rc.category_id
AND             rc.resource_id = pr.resource_id
AND             pr.product_id = p.id
AND             p.id = pe.product_id
AND             pe.ebook_id = e.id
AND             c.id = category1_.id as formula1_1_,
                subject2_.id         AS id1_102_2_,
                subject2_.NAME       AS name2_102_2_
FROM            resource_category categories0_
INNER JOIN      category category1_
ON              categories0_.category_id=category1_.id
LEFT OUTER JOIN subject subject2_
ONSELECT DISTINCT e.subject_id
FROM            category c,
                resource_category rc,
                product_resource pr,
                product p,
                product_ebook pe,
                book e
WHERE           c.id = rc.category_id
AND             rc.resource_id = pr.resource_id
AND             pr.product_id = p.id
AND             p.id = pe.product_id
AND             pe.ebook_id = e.id
AND             c.id = category1_.id=subject2_.id
where           categories0_.resource_id=?

Notice the "left outer join subject subject2_ on SELECT DISTINCT e.subject_id" and "AND c.id = category1_.id=subject2_.id" towards the end.

Edit 2:

Here is the entity involved in the above query

@Entity
@Table(name="resource")
public class Resource implements Serializable, Comparable<Resource>
{
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(generator = "uuid-strategy")
    private String id;

    …    
    @Column(name = "FILE_NAME")
    private String fileName;

    @Column(name = "URI")
    private String uri;

    …

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "resource_category", joinColumns = { @JoinColumn(name = "RESOURCE_ID") }, inverseJoinColumns = { @JoinColumn(name = "CATEGORY_ID") })
    private Set<Category> categories;

and here is the query itself …

CriteriaBuilder builder = m_entityManager.getCriteriaBuilder();
CriteriaQuery<T> criteria = builder.createQuery(Resource.class);
Root<T> rootCriteria = criteria.from(Resource.class);
criteria.select(rootCriteria).where(builder.equal(rootCriteria.get(“uri”),uri));
Resource ret = null;
try {
        final TypedQuery<T> typedQuery = m_entityManager.createQuery(criteria);
        ret = typedQuery.getSingleResult();
} catch (NoResultException e) {
        LOG.warn(e.getMessage());
}
return ret; 

Answer 1

You need to use Hibernate specific JoinColumnOrFormula:

public class Category implements Serializable, Comparable<Category> {
    private static final long serialVersionUID = 1L;

    @Id
    @Column(name = "ID")
    @GeneratedValue(generator = "uuid-strategy")
    private String id;

    @NotEmpty
    @ManyToOne
    @JoinColumnsOrFormulas({
        @JoinColumnOrFormula(
            formula = @JoinFormula(
                value = 
                    "SELECT DISTINCT e.subject_id " +
                    "FROM category c, resource_category rc, product_resource pr, " +
                    "     sb_product p, product_book pe, book e " +
                    "WHERE c.id = rc.category_id " +
                    "  AND rc.resource_id = pr.resource_id " +
                    "  AND pr.product_id = p.id " +
                    "  AND p.id = pe.product_id " +
                    "  AND pe.ebook_id = e.id " +
                    "  AND c.id = ‘ABCEEFGH‘", 
                referencedColumnName="id"
            )
        )
    })
    private Set<Subject> subjects;

    ...
}

Or, you can include this query in a stored procedure:

CREATE FUNCTION join_book(text) RETURNS text
    AS  'SELECT DISTINCT e.subject_id ' +
        'FROM category c, resource_category rc, product_resource pr, ' +
        '     sb_product p, product_book pe, book e ' +
        'WHERE c.id = rc.category_id ' +
        '  AND rc.resource_id = pr.resource_id ' +
        '  AND pr.product_id = p.id ' +
        '  AND p.id = pe.product_id ' +
        '  AND pe.ebook_id = e.id ' +
        '  AND c.id = $1;'  
    LANGUAGE SQL
    IMMUTABLE
    RETURNS NULL ON NULL INPUT;

And, then your mapping becomes:

public class Category implements Serializable, Comparable<Category> {
    private static final long serialVersionUID = 1L;

    @Id
    @Column(name = "ID")
    @GeneratedValue(generator = "uuid-strategy")
    private String id;

    @NotEmpty
    @ManyToOne
    @JoinColumnsOrFormulas({
        @JoinColumnOrFormula(
            formula = @JoinFormula(
                value = "join_book(id)", 
                referencedColumnName="id"
            )
        )
    })
    private Set<Subject> subjects;

    ...
}