This modules support index-time and query-time joins.

## Index-time joins

The index-time joining support joins while searching, where joined documents are indexed as a single document block using {@link org.apache.lucene.index.IndexWriter#addDocuments}. This is useful for any normalized content (XML documents or database tables). In database terms, all rows for all joined tables matching a single row of the primary table must be indexed as a single document block, with the parent document being last in the group.

When you index in this way, the documents in your index are divided into parent documents (the last document of each block) and child documents (all others). You provide a {@link org.apache.lucene.search.Filter} that identifies the parent documents, as Lucene does not currently record any information about doc blocks.

At search time, use {@link org.apache.lucene.search.join.ToParentBlockJoinQuery} to remap/join matches from any child {@link org.apache.lucene.search.Query} (ie, a query that matches only child documents) up to the parent document space. The resulting query can then be used as a clause in any query that matches parent.

If you only care about the parent documents matching the query, you can use any collector to collect the parent hits, but if you'd also like to see which child documents match for each parent document, use the {@link org.apache.lucene.search.join.ToParentBlockJoinCollector} to collect the hits. Once the search is done, you retrieve a {@link org.apache.lucene.search.grouping.TopGroups} instance from the {@link org.apache.lucene.search.join.ToParentBlockJoinCollector#getTopGroups} method.

To map/join in the opposite direction, use {@link org.apache.lucene.search.join.ToChildBlockJoinQuery}. This wraps any query matching parent documents, creating the joined query matching only child documents.

## Search-time joins

The query time joining is index term based and implemented as two pass search. The first pass collects all the terms from a fromField that match the fromQuery. The second pass returns all documents that have matching terms in a toField to the terms collected in the first pass.

Query time joining has the following input:

- `fromField`: The from field to join from.
- `fromQuery`: The query executed to collect the from terms. This is usually the user specified query.
- `toField`: The to field to join to

Basically the query-time joining is accessible from one static method. The user of this method supplies the method with the described input and a `IndexSearcher` where the from terms need to be collected from. The returned query can be executed with the same `IndexSearcher`, but also with another `IndexSearcher`. Example usage of the {@link org.apache.lucene.search.join.JoinUtil#createJoinQuery(String, String, org.apache.lucene.search.Query, org.apache.lucene.search.IndexSearcher)} :

      String fromField = "from"; // Name of the from field
      String toField = "to"; // Name of the to field
      Query fromQuery = new TermQuery(new Term("content", searchTerm)); // Query executed to collect from values to join to the to values

      Query joinQuery = JoinUtil.createJoinQuery(fromField, toField, fromQuery, fromSearcher);
      TopDocs topDocs = toSearcher.search(joinQuery, 10); // Note: toSearcher can be the same as the fromSearcher
      // Render topDocs...