Question

I’m trying to put together a complicated query using Hibernate. I’ve been leaning toward Criteria, but I’m beginning to suspect it’s not possible, and so any suggestions would be helpful.

I have an entity structure like the following:

public class Attribute {
    private Integer id;
    private String name;
    private Set<Value> values;
}

public class Instance {
    private Integer id;
    private int instanceRef;
    private Set<Value> values;
}

public class Value {
    private Integer id;
    private Attribute attribute;
    private String localAttributeName;
    private Instance instance;
    private String value;
}

These entities are related as you’d expect:

value.attribute_id --> attribute.id
value.instance_id --> instance.id

Now, I would like to be able to take a set of attribute/value pairs (Strings) and find all instances that contain all of them. In Value, only one of attribute and localAttributeName are non-null, so the attribute name may match either localAttributeName or attribute.name. And to complicate things one last time, the unique index on Value is on (instance, attribute, value) or (instance, localAttributeName, value) — that is, within an Instance, any given Attribute may have multiple Values.

This is what I have so far:

public List<Instance> getMatchingInstances(Map<String, String> attrValues) {
    Criteria crit = session.createCriteria(Instance.class, "i");
    for(Map.Entry<String, String> entry : attrValues) {
        DetachedCriteria valueCrit = DetachedCriteria.forClass(Value.class, "v");

        // Do something here with valueCrit

        crit.add(Subqueries.exists(valueCrit));
    }
    return crit.list();
}

Based on the research I’ve done, what I’ve tried for that Do something section is:

    // This would only check localAttributeName and not attribute.name.
    // That's okay -- once I get the rest to work, I can figure this out.
    valueCrit.add(Restrictions.eq("localAttributeName", entry.getKey());
    valueCrit.add(Restrictions.eq("value", entry.getValue());
    valueCrit.add(Restrictions.eqProperty("v.instance_id", "i.id"));

But this throws the exception below, which I suspect is telling me I can’t do this with Criteria, but I’d love to learn otherwise:

java.lang.NullPointerException
    at org.hibernate.loader.criteria.CriteriaQueryTranslator.getProjectedTypes(CriteriaQueryTranslator.java:341)

What would be the best way to go about doing this?

Answer 1

I figured out the solution after a few hours of banging on it. Hopefully, this is of use to others. There were three main points that I needed to solve to make this feasible:

1. Add a Projection
2. Create the proper joins
3. Properly map the subquery back to the main criteria

I’ve highlighted each of these in the below code.

First, to get rid of the exception, I discovered that the subquery needed a projection, highlighted below. I just did a projection on the “id” property of Instance.

Second, to get the join, I used the Criteria.createCriteria() methods to create a left outer join. Because I had multiple conditions at different levels of the join, I had to save the joined Criteria and attach expressions to them separately. This let me do my OR expression in the subquery.

Finally, I had to add an eqProperty() clause to map the subquery back to the main Criteria. Just like it would need to be in the resulting SQL, I used: instance.id = i.id. Because I had already mapped the Instance Criteria to “i” and was adding this clause to the Value Criteria, this translated to the SQL: v.instance_id = i.id.

Here’s the working code:

public List<Instance> getMatchingInstances(Map<String, String> attrValues) {
    Criteria crit = session.createCriteria(Instance.class, "i");
    for(Map.Entry<String, String> entry : attrValues) {
        String attrName = entry.getKey();
        String val = entry.getValue();

        // Create the subquery
        DetachedCriteria valueCrit = DetachedCriteria.forClass(Value.class, "v");

        // Join the Attribute object (left outer join)
        DetachedCriteria attrCrit = 
          valueCrit.createCriteria("attribute", CriteriaSpecification.LEFT_JOIN);

        // Put together the OR statement on the Attribute joined criterion.
        Criterion localAttr = Restrictions.eq("v.localAttributeName", attrName);
        Criterion globalAttr = Restrictions.eq("name", attrName);
        attrCrit.add(Restrictions.or(localAttr, globalAttr));

        // Simple column equality on the subquery criterion.
        valueCrit.add(Restrictions.eq("value", val));

        // Map the subquery back to the outer query.
        valueCrit.add(Restrictions.eqProperty("instance.id", "i.id"));

        // Add the missing projection.
        valueCrit.setProjection(Projections.property("id"));

        // Add this subquery to the outer query.
        crit.add(Subqueries.exists(valueCrit));
    }
    return crit.list();
}