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This plugin provides hierarchical facets for Blacklight.

Please note this is does not directly follow any of the competing approaches of Hierarchical Facets in Solr, including Solr PivotFacets.


Add the plugin to your Blacklight app's Gemfile.

gem 'blacklight-hierarchy'

Index your hierarchies in a (colon-)separated list. For example, items in a "processing" queue with a "copy" action, might be indexed as:

  <field name="id">foo</field>
  <field name="queue_status_facet">processing</field>
  <field name="queue_status_facet">processing:copy</field>
  <field name="queue_status_facet">processing:copy:waiting</field>
  <field name="id">bar</field>
  <field name="queue_status_facet">processing</field>
  <field name="queue_status_facet">processing:copy</field>
  <field name="queue_status_facet">processing:copy:completed</field>

That would cause the facet count to appear at all three levels:

You can skip as many levels as you'd like, as long as the "leaf" values are indexed. For example, if you didn't index the "processing" part alone, it will simply be a container, not a clickable/countable facet:

Note: If you use Solr's built-in PathHierarchyTokenizerFactory, you can index the entire depth by supplying only the leaf nodes. Otherwise you are expected to build the permutations yourself before loading.

In your Blacklight controller configuration (usually CatalogController), tell Blacklight to render the facet using the hierarchy component.

config.add_facet_field 'queue_wps',   label: 'Queue Status', component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'queue_wsp',   label: 'Queue Status', component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'queue_swp',   label: 'Queue Status', component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'callnum_top', label: 'Callnumber',   component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'foo_trunk',   label: 'Foo L1',       component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'foo_branch',  label: 'Foo L2',       component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'foo_leaves',  label: 'Foo L3',       component: Blacklight::Hierarchy::FacetFieldListComponent
config.add_facet_field 'tag_facet',   label: 'Tag',          component: Blacklight::Hierarchy::FacetFieldListComponent

Add your hierarchy-specific options to the controller configuration:

config.facet_display = {
  :hierarchy => {
    'queue'   => [['wps','wsp','swp'], ':'],       # values are arrays: 1st element is array, 2nd is delimiter string
    'callnum' => [['top'], '/'],
    'foo'     => [['trunk', 'branch', 'leaves']],  # implied default delimiter
    'tag'     => [[nil]]                           # TODO: explain

In the above configuration, 'queue_status_facet' is the full Solr field name, and ':' is the delimiter within the field. Note that suffixes ('facet' in the above example) should not contain underscores, since the methods that deal with the Solr fields and match them to the config assume the "prefix" ('queue_status' in the above example) will be everything up to the last underscore in the field name. See the facet_tree method for further explanation and some relevant code, as well as the render_hierarchy method for relevant code.

The [nil] value is present in support of rotatable facet hierarchies, a totally undocumented feature.

Facet fields should be added for each permutation of hierarchy key and term values, joined by _. Or, the output of:

config.facet_display[:hierarchy].each{ |k,v| puts "#{k}_#{v}" }

Overriding the icon

The icon is available in an engine configuration. You can change them in an initializer in your app.

Blacklight::Hierarchy::Engine.config.closed_icon = ''
Blacklight::Hierarchy::Engine.config.opened_icon = ''

Aria Labels

For screen reader purposes we have used "Toggle subgroup" as the aria-label attribute of the button. This is internationalized using rails' i18n feature.

The field name is used in the key to allow for facet specific aria labels or defaults back to the generic key/"Toggle subgroup" text.

# config/locales/en.yml
      format_ssim_toggle_aria_label: Toggle format section
      toggle_aria_label: Toggle call number section


The javascript in this project requires jquery, but it's up to you to provide it in a way that best works for your project. You may consider the jquery-rails gem or if you use webpacker, you could use the jquery npm package.


This code was ripped out of another project, and is still quite immature as a standalone project. Every effort has been made to make it as plug-and-play as possible, but it may stomp on Blacklight in unintended ways (e.g., ways that made sense in context of its former host app, but which aren't compatible with generic Blacklight). Proceed with caution, and report issues.


  • Switch internal facet management from hack-y Hash to Blacklight::Hierarchy::FacetGroup class (already implemented, but not plumbed up)
  • Add configuration support for hierarchy delimiters other than /\s*:\s*/ (baked into Blacklight::Hierarchy::FacetGroup, but again, requiring additional plumbing)
  • Clarify when suffix is applied/required/etc.