= BlockJoin Faceting
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BlockJoin facets allow you to aggregate children facet counts by their parents.

It is a common requirement that if a parent document has several children documents, all of them need to increment facet value count only once. This functionality is provided by `BlockJoinDocSetFacetComponent`, and `BlockJoinFacetComponent` just an alias for compatibility.

CAUTION: This component is considered experimental, and must be explicitly enabled for a request handler in `solrconfig.xml`, in the same way as any other <<requesthandlers-and-searchcomponents-in-solrconfig.adoc#requesthandlers-and-searchcomponents-in-solrconfig,search component>>.

This example shows how you could add this search components to `solrconfig.xml` and define it in request handler:

[source,xml]
----
  <searchComponent name="bjqFacetComponent" class="org.apache.solr.search.join.BlockJoinDocSetFacetComponent"/>

   <requestHandler name="/bjqfacet" class="org.apache.solr.handler.component.SearchHandler">
    <lst name="defaults">
      <str name="shards.qt">/bjqfacet</str>
    </lst>
    <arr name="last-components">
      <str>bjqFacetComponent</str>
    </arr>
  </requestHandler>
----

This component can be added into any search request handler. This component work with distributed search in SolrCloud mode.

Documents should be added in children-parent blocks as described in <<uploading-data-with-index-handlers.adoc#nested-child-documents,indexing nested child documents>>. Examples:

.Sample document
[source,xml]
----
<add>
  <doc>
    <field name="id">1</field>
    <field name="type_s">parent</field>
    <doc>
      <field name="id">11</field>
      <field name="COLOR_s">Red</field>
      <field name="SIZE_s">XL</field>
      <field name="PRICE_i">6</field>
    </doc>
    <doc>
      <field name="id">12</field>
      <field name="COLOR_s">Red</field>
      <field name="SIZE_s">XL</field>
      <field name="PRICE_i">7</field>
    </doc>
    <doc>
      <field name="id">13</field>
      <field name="COLOR_s">Blue</field>
      <field name="SIZE_s">L</field>
      <field name="PRICE_i">5</field>
    </doc>
  </doc>
  <doc>
    <field name="id">2</field>
    <field name="type_s">parent</field>
    <doc>
      <field name="id">21</field>
      <field name="COLOR_s">Blue</field>
      <field name="SIZE_s">XL</field>
      <field name="PRICE_i">6</field>
    </doc>
    <doc>
      <field name="id">22</field>
      <field name="COLOR_s">Blue</field>
      <field name="SIZE_s">XL</field>
      <field name="PRICE_i">7</field>
    </doc>
    <doc>
      <field name="id">23</field>
      <field name="COLOR_s">Red</field>
      <field name="SIZE_s">L</field>
      <field name="PRICE_i">5</field>
    </doc>
  </doc>
</add>
----

Queries are constructed the same way as for a <<other-parsers.adoc#block-join-query-parsers,Parent Block Join query>>. For example:

[source,text]
----
http://localhost:8983/solr/bjqfacet?q={!parent which=type_s:parent}SIZE_s:XL&child.facet.field=COLOR_s
----

As a result we should have facets for Red(1) and Blue(1), because matches on children `id=11` and `id=12` are aggregated into single hit into parent with `id=1`.

The key components of the request shown above are:

`/bjqfacet?`::
The name of the request handler that has been defined with a block join facet component enabled.

`q={!parent which=type_s:parent}SIZE_s:XL`::
The mandatory parent query as a main query. The parent query could also be a subordinate clause in a more complex query.

`&child.facet.field=COLOR_s`::
The child document field, which might be repeated many times with several fields, as necessary.