How to query for empty collection in hibernate search - hibernate-search

Question

We are using hibernate-search with ES back end. Parent class has multiple collection of different children objects. We can filter parents based on the value of the child as the collections are annotated with #IndexedEmbedded.
We want to be able to filter parents based on if the collection of children is empty or not. We have tried using #IndexedEmbedded(indexNullAs = "null"), and then filtering on queryBuilder.phrase().onField("parent.children").sentence("null").createQuery(), which has made no difference. Using the ES console we can show the parent, but when the collection is empty it isn't listed at all, leading us to believe it hasn't been indexed due to it being empty.
Another option would be to filter on parent.collection.field using a wildcard, however this would not be recommended by the hibernate search docs due to the performance.

Answer 1

If you upgrade to Hibernate Search 6, you will be able to use the exists predicate:
List<MyEntity> hits = Search.session(entityManager)
.search(MyEntity.class)
.where(f -> f.matchAll().except(f.exists().field(“parent.children”))
.fetchHits(20);
That would solve your problem, and then you can start worrying about performance in your specific case.
Still in Hibernate Search 6, if your tests show that performance of the first solution really is problematic, I would suggest using a custom bridge on children that indexes whether the collection is empty or not. Something like this:
#SuppressWarnings("rawtypes")
public class MyCollectionEmptyBridge implements ValueBridge<Collection, Boolean> {
#Override
public Boolean toIndexedValue(Collection value, ValueBridgeToIndexedValueContext context) {
return value == null || value.isEmpty();
}
}
public class MyParentEntity {
// ...
#GenericField(
name = "childrenAreEmpty",
valueBridge = #ValueBridgeRef(type = MyCollectionEmptyBridge.class),
// Apply the bridge directly to the collection and not to its elements
// See https://docs.jboss.org/hibernate/stable/search/reference/en-US/html_single/#_disabling_container_extraction
extraction = #ContainerExtraction(extract = ContainerExtract.NO)
)
private List<Child> children = new ArrayList<>();
}
List<MyEntity> hits = Search.session(entityManager)
.search(MyEntity.class)
.where(f -> f.match().field(“parent.childrenAreEmpty”).matching(true))
.fetchHits();
That second solution can be implemented with Hibernate Search 5 as well, though Hibernate Search 5's custom bridges are somewhat harder to work with.