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Querying with Hibernate Search
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By default, objects returned by a full-text query are provided by descending score: The most relevant object is returned first. While this will satisfy your needs in most cases, returning objects ordered by one or several properties is sometimes more pertinent for your user. You can do that with Hibernate Search. Sort is applied to the Lucene query (on the value stored in Lucene fields) rather than to the database query. To order using a database query, Hibernate Search would need to read and extract the identifiers for all matching documents (potentially millions!) and create an SQL query from them. This would be very inefficient in most cases. Hibernate Search delegates the sort operation to Lucene. Fields used for the sort have one restriction: They must use either one of these two indexing strategies: Index.UN_TOKENIZED or Index.NO_NORMS. Remember, you can map the same object property multiple times: once to build a sortable field, once to build a searchable field (see section 3.3.4). More than once you will index a field specifically for sorting, even if you re not interested in searching by it. Sort is described by a Lucene Sort object, which contains the list of fields to sort by, each field sort being represented by a SortField object. Let s look at a simple example in listing 6.20. Each field must be indexed but not tokenized.
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Listing 6.20 Sorting results by a set of fields
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@Entity @Indexed public class Item { @Id @GeneratedValue @DocumentId private Integer id; @Fields( {@Field(index=Index.TOKENIZED, store=Store.YES), @Field(name="title_sort", index=Index.UN_TOKENIZED), Properties untokenized }) can be sorted @NotNull @Length(max=100) private String title;
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@Field @Length(max=5000) private String description; @Field(index=Index.UN_TOKENIZED, store=Store.YES)@Length(max=16) private String ean; Properties untokenized ... can be sorted } public List<String> displayAllByMatchingTitleOrderedBy(String words, OrderBy orderBy) { org.apache.lucene.search.Query luceneQuery = buildLuceneQuery( words, Item.class );
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FullTextSession ftSession = SessionHolder.getFullTextSession(); FullTextQuery query = ftSession.createFullTextQuery( luceneQuery, Item.class ); Sort sort = null; switch (orderBy) {
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ean case EAN: SortField sortField = new SortField( "ean", SortField.STRING ); sort = new Sort( sortField ); break; Sort by title,
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Build a SortField
Wrap it in a Sort
then by ean Multiple sort case TITLE_THEN_EAN: fields are possible SortField[] sortFields = new SortField[2]; sortFields[0] = new SortField( "title_sort", SortField.STRING ); sortFields[1] = new SortField( "ean", SortField.STRING ); sort = new Sort( sortFields ); break;
default: assert sort == null : "Unknown OrderBy." + orderBy; }
query.setSort( sort );
Assign Sort to the query
@SuppressWarnings("unchecked") List<Item> items = query.list(); List<String> results = new ArrayList<String>(); for (Item item : items) { StringBuilder itemInString = new StringBuilder( "Item " ) .append( item.getTitle() ) .append( " (" ).append( item.getEan() ).append( ")" ); results.add( itemInString.toString() ); } return results; }
B A sortable version of title is indexed in title_sort in the field UN_TOKENIZED. C ean is already sortable. D A SortField object takes the field name and the sort style. E Sort receives the SortField objects; the order in which they are provided to Sort is significant. F You can sort by multiple fields. You can sort by title_sort, and for identical title_sort values you can sort by ean. G Sort is associated with the FullTextQuery object.
The sort algorithm differs slightly depending on the property type. Each SortField can define its type. The available types are:
SortField.INT Integer comparator SortField.LONG Long comparator SortField.FLOAT Float comparator SortField.DOUBLE Double comparator SortField.STRING String comparator SortField.CUSTOM Custom comparator
Querying with Hibernate Search
WARNING Lucene supports SortField.AUTO, which determines the type based
on the first result. The authors recommend against using it because it can have unintended consequences. What if the first result looks like an int but turns out to be a long or a String Sorting by field is not free. In order to sort data, Lucene needs to load and keep the field ordering. Lucene uses some tricks to keep ordering for numeric values efficient in terms of memory usage. String and custom field sorting cannot benefit from these optimizations. Make sure you test sort on a real dataset and measure memory consumption. Beyond field-based sorting, Lucene allows you to sort by score (descending) and by Lucene document id (ascending), using respectively the following special SortField instances:
SortField.FIELD_SCORE Sort documents by their score (highest score first). SortField.FIELD_DOC Sort documents by Lucene document id (lowest first).
Listing 6.21 extends listing 6.20 by supporting sorting by title and score and makes use of these special fields.
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