[njakobsen]

Adds child_facet: a facet on a field of nested children that counts their parents. A search can already filter parents by their children (with_child) and sort by them (order_by_child). This adds the missing piece: listing the values the children of the matching parents have, restricted to the children meeting some conditions. The use it's built for is suggesting the next value of a nested filter in Glint's search autocomplete.

search = Project.search do
  with :status, 'active'
  child_facet :milestones, :name, :matches => 'des.*' do
    with :owner_names, 'ana'
  end
end
search.facet(:name).rows # each row's count is a number of projects, not milestones

What changes

• DSL::FieldQuery#child_facet(association, field, options = {}, &block) adds a JSON terms facet:
  • Its domain is the blockChildren of the parents matching the search.
  • Its filter is the same child restrictions with_child builds from the block. BlockJoin#child_query is now public so both use one definition.
  • A uniqueBlock(_root_) sub-facet named distinct makes each row count parents. JsonFacetRow already reads a distinct sub-facet as the row's count.
• :matches keeps only the children whose value matches a Lucene regular expression as a whole. JSON facets have no facet.matches, so it goes in the domain filter. The other json_facet options apply too.
• The result is read with facet, by the field's name or by :name.
• The README documents it beside order_by_child.

Solr 8 and above, for uniqueBlock. The new examples skip on older Solr, as the order_by_child examples do. I chose 8 as the cut-off, but only tested 9.10.1.

Testing

• New examples in spec/integration/nested_documents_spec.rb:
  • It counts parents, not children.
  • It counts only the children meeting the block's restrictions.
  • It counts only the children of parents matching the search.
  • :matches, including a pattern containing a slash.
• Red→green:
  • Without the uniqueBlock sub-facet, a project with two "build" milestones counts 3 instead of 2.
  • Without the block's restrictions, another owner's milestone is counted.
• Full sunspot suite against solr:9.10.1 with ci/solr9: 1,542 examples, 0 failures, 6 pending.

Draft until the Glint autocomplete work that uses it is ready.

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