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Lucene 3.6 introduces the notion of "Query Time Join": a way to relate Documents from different indexes and filter content and retrieve fields. This approach comes at a runtime costs as an extra pass is involved in processing the query.
The idea is basically that if you search on e.g. Posts {{Post}} instances and you need the photo of the {{ User }} that is part of the {{ Post }} , you can keep this information separate and retrieve the {{ User }} on the fly. This way you can ensure that fields that change in the {{ User }} don't require a reindexing re-indexing of all the related {{ Comments .
}} - http://www.searchworkings.org/blog/-/blogs/412000
Query time joining in Lucene is pretty straight forward, and entirely encapsulated in {{ JoinUtil.createJoinQuery }} . It requires the following arguments:
fromField. * _fromField_ - The entity field to join in the entity being queried: e.g. user.id
toField. * _toField_ - The entity field in the related index to join on: e.g. id.
fromQuery. * _fromQuery_ - The query executed to collect the from terms.
fromSearcher. * _fromSearcher_ - The search on where the fromQuery is executed.
multipleValuesPerDocument. * _multipleValuesPerDocument_ - Whether the {{ fromField }} contains more than one value per document ( multivalued multi-valued field). If this option is set to {{ true }} the from terms can be collected in a more efficient manner.
Since this doesn't require indexing changes and just affects what is returned, it can simply be implemented as an extension to the querybuilder {{QueryBuilder}} .
{code}
QueryBuilder qbGroup = fts.getSearchFactory().buildQueryBuilder().forEntity(DiscussionGroup.class).get();
QueryBuilder qbPost = fts.getSearchFactory().buildQueryBuilder().forEntity(Post.class).get();
Query joinQuery = qbGroup.join().onFields("user.id","id").must(
qbGroup.keyword().onField("title").matching("lucene").createQuery()
).createQuery();
qbPost.bool().must(
joinQuery
).createQuery();
{code}
I'm not sure at this point but I believe that Query Joining query joining doesn't actually retrieve the related document. Which would be a nice feature also.
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