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Listing 6.17 Projecting metadata fields
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public List<ItemView> displayProjectionAndMetadataByMatchingTitle(String words) { FullTextSession ftSession = SessionHolder.getFullTextSession(); org.apache.lucene.search.Query luceneQuery = buildLuceneQuery( words, Item.class ); FullTextQuery query = ftSession.createFullTextQuery( luceneQuery, Item.class ); @SuppressWarnings("unchecked") List<Object[]> results = query .setProjection( "ean", "title", FullTextQuery.SCORE) .list();
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Project the document score
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List<ItemView> endResults = new ArrayList<ItemView>( results.size() ); for (Object[] line : results) { ItemView itemView = new ItemView( (String) line[0], Retrieve the (String) line[1], document score (Float) line[2] ); endResults.add( itemView ); } return endResults;
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B Metadata is projected along with regular properties using their constant placeholders. C Score is available at the expected position.
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Some might complain that playing with Object[] as a returned element is not the easiest thing on earth. As a matter of fact, in listing 6.16 we convert the Object[] into an ItemView object manually. Hibernate Search allows you to transform the structure before returning it by the query.
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Manipulating the result structure
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One common use case for projection is to display two or three fields of the matching results and expose them as view objects. A view object is a read-only object used to convey data from the backend layer to the frontend layer for display. Typically, the
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Querying with Hibernate Search
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view object structure is different from the raw domain model structure (some aggregation or preparation is performed on fields, and so on). Hibernate Search queries let you transform the projection s Object[] structure into the structure of your choice by providing a ResultTransformer API. This API has two methods; the most interesting one is transformTuple, which takes the Object[] and returns the structure of your choice. Listing 6.18 demonstrates a ResultTransformer returning a Map<String, Object> containing the property name projected as a key and the property value as a value. A ResultTransformer instance can be set on a FullTextQuery (based on the Hibernate Core or Java Persistence API).
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Listing 6.18
ResultTransformer converting Object[] into a Map<String,
public class ProjectionToMapResultTransformer implements ResultTransformer { public Object transformTuple( Object[] tuple, String[] aliases) {
Method called for each result
Map result = new HashMap( tuple.length ); for (int i = 0; i < tuple.length; i++) { String key = aliases[i]; if ( key != null ) { result.put( key, tuple[i] ); } } return result; } public List transformList(List collection) { return collection; } } List<Map<String, Object>> results = fullTextQuery Attach the result .setProjection( "ean", "title" ) transformer .setResultTransformer( new ProjectionToMapResultTransformer() ) .list();
B transformTuple() takes the Object[] returned by a regular projection query and the associated aliases and returns the data in a different structure. B The Query
instance is configured with the ResultTransformer object. Two ResultTransformer objects that are quite useful are provided by Hibernate out of the box:
AliasToBeanResultTransformer AliasToBeanConstructorResultTranformer
The first one injects projected properties into the setter of the same name (or the field of the same name). The second one injects projected properties through a specific constructor. See listing 6.19.
Manipulating the result structure
Listing 6.19
Using AliasToBeanResultTransformer to populate a view object
public class ItemView { private String ean; private String title;
Projected property matches the attribute
public String getEan() { return ean; } public String getTitle() { return title; } public float getScore() { return score; } } public List<ItemView> displayProjectionUsingResultTransformerByMatchingTitle(String words) { FullTextSession ftSession = SessionHolder.getFullTextSession(); org.apache.lucene.search.Query luceneQuery = buildLuceneQuery( words, Item.class ); FullTextQuery query = ftSession.createFullTextQuery( luceneQuery, Item.class ); @SuppressWarnings("unchecked") List<ItemView> results = query Attach the result .setProjection( "ean", "title" ) transformer .setResultTransformer( new AliasToBeanResultTransformer( ItemView.class ) ) .list(); return results;
B Field names (or setters) match the property names projected; the projected values are stored in their respective field or getter in ItemView. C The ResultTransformer
instance is passed to the query; the query now returns a List<ItemView>.
WARNING Objects built by a ResultTransformer are not managed by the persis-
tence context. Hibernate provides additional ResultTransformers that can be found in the org.hibernate.transform package, but this interface is meant to be implemented by applications to fit their needs. Don t limit yourself to the list of built-in ones. We ve just shown how to adjust the object-retrieval process and especially how to retrieve a custom structure from a projection query. Stepping back, it would be nice to customize the order in which objects are returned to us. Read on.
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