## org.apache.lucene.search.join
Class ToParentBlockJoinCollector

    java.lang.Object
      org.apache.lucene.search.Collector
          org.apache.lucene.search.join.ToParentBlockJoinCollector

    public class ToParentBlockJoinCollector

extends Collector

Collects parent document hits for a Query containing one more more BlockJoinQuery clauses, sorted by the specified parent Sort. Note that this cannot perform arbitrary joins; rather, it requires that all joined documents are indexed as a doc block (using `IndexWriter.addDocuments(java.util.Collection<org.apache.lucene.document.Document>)` or `IndexWriter.updateDocuments(org.apache.lucene.index.Term, java.util.Collection<org.apache.lucene.document.Document>)`). Ie, the join is computed at index time.

The parent Sort must only use fields from the parent documents; sorting by field in the child documents is not supported.

You should only use this collector if one or more of the clauses in the query is a `ToParentBlockJoinQuery`. This collector will find those query clauses and record the matching child documents for the top scoring parent documents.

Multiple joins (star join) and nested joins and a mix of the two are allowed, as long as in all cases the documents corresponding to a single row of each joined parent table were indexed as a doc block.

For the simple star join you can retrieve the `TopGroups` instance containing each `ToParentBlockJoinQuery`'s matching child documents for the top parent groups, using `getTopGroups(org.apache.lucene.search.join.ToParentBlockJoinQuery, org.apache.lucene.search.Sort, int, int, int, boolean)`. Ie, a single query, which will contain two or more `ToParentBlockJoinQuery`'s as clauses representing the star join, can then retrieve two or more `TopGroups` instances.

For nested joins, the query will run correctly (ie, match the right parent and child documents), however, because TopGroups is currently unable to support nesting (each group is not able to hold another TopGroups), you are only able to retrieve the TopGroups of the first join. The TopGroups of the nested joins will not be correct. See `org.apache.lucene.search.join` for a code sample.

WARNING: This API is experimental and might change in incompatible ways in the next release.

Constructor Summary
`ToParentBlockJoinCollector(Sort sort, int numParentHits, boolean trackScores, boolean trackMaxScore)`

Method Summary
`boolean` `acceptsDocsOutOfOrder()` Return `true` if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to `Collector.collect(int)`.
`void` `collect(int parentDoc)` Called once for every document matching a query, with the unbased document number.
`TopGroups<Integer>` `getTopGroups(ToParentBlockJoinQuery query, Sort withinGroupSort, int offset, int maxDocsPerGroup, int withinGroupOffset, boolean fillSortFields)` Return the TopGroups for the specified BlockJoinQuery.
`void` `setNextReader(IndexReader reader, int docBase)` Called before collecting from each IndexReader.
`void` `setScorer(Scorer scorer)` Called before successive calls to `Collector.collect(int)`.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### ToParentBlockJoinCollector

    public ToParentBlockJoinCollector(Sort sort,
                                      int numParentHits,
                                      boolean trackScores,
                                      boolean trackMaxScore)
                               throws IOException

Throws:
  `IOException`

Method Detail

### collect

    public void collect(int parentDoc)
                 throws IOException

  Description copied from class: `Collector`
  Called once for every document matching a query, with the unbased document number.

  Note: This is called in an inner search loop. For good search performance, implementations of this method should not call `Searcher.doc(int)` or `IndexReader.document(int)` on every hit. Doing so can slow searches by an order of magnitude or more.

  Specified by:
    `collect` in class `Collector`
  Throws:
    `IOException`

### setNextReader

    public void setNextReader(IndexReader reader,
                              int docBase)
                       throws IOException

  Description copied from class: `Collector`
  Called before collecting from each IndexReader. All doc ids in `Collector.collect(int)` will correspond to reader. Add docBase to the current IndexReaders internal document id to re-base ids in `Collector.collect(int)`.

  Specified by:
    `setNextReader` in class `Collector`
  Parameters:
    `reader` - next IndexReader
  Throws:
    `IOException`

### acceptsDocsOutOfOrder

    public boolean acceptsDocsOutOfOrder()

  Description copied from class: `Collector`
  Return `true` if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to `Collector.collect(int)`.

  Most Lucene Query implementations will visit matching docIDs in order. However, some queries (currently limited to certain cases of `BooleanQuery`) can achieve faster searching if the `Collector` allows them to deliver the docIDs out of order.

  Many collectors don't mind getting docIDs out of order, so it's important to return `true` here.

  Specified by:
    `acceptsDocsOutOfOrder` in class `Collector`

### setScorer

    public void setScorer(Scorer scorer)

  Description copied from class: `Collector`
  Called before successive calls to `Collector.collect(int)`. Implementations that need the score of the current document (passed-in to `Collector.collect(int)`), should save the passed-in Scorer and call scorer.score() when needed.

  Specified by:
    `setScorer` in class `Collector`

### getTopGroups

    public TopGroups<Integer> getTopGroups(ToParentBlockJoinQuery query,
                                           Sort withinGroupSort,
                                           int offset,
                                           int maxDocsPerGroup,
                                           int withinGroupOffset,
                                           boolean fillSortFields)
                                    throws IOException

  Return the TopGroups for the specified BlockJoinQuery. The groupValue of each GroupDocs will be the parent docID for that group. Note that the `GroupDocs.totalHits`, which would be the total number of child documents matching that parent, is not computed (will always be 0). Returns null if no groups matched.

  Throws:
    `IOException`