added facet source to the field formatter config

README.md

@@ -21,6 +21,14 @@ query is resolved per field type:
 This matches how such fields are normally indexed and faceted in Solr — make
 sure the facet you select indexes the same readable value.
 
+You can override this per field with the **Facet value** setting:
+
+- **Automatic** (default): referenced entity label for reference fields, the
+  field value otherwise.
+- **Referenced entity label**: always use the referenced entity's label.
+- **Raw stored value**: use the stored value — the entity id for reference
+  fields. Choose this when your facet indexes entity ids rather than labels.
+
 ## Dependencies
 
 - `search_api_solr`
@@ -38,6 +46,8 @@ The formatter is configured per field, under **Manage display** for the entity
 - **Facet:** the facet to filter on. Pick the facet configured for this field
   on the chosen search page. Its URL alias is used as the facet key in the
   generated query.
+- **Facet value:** which value of the field to filter by. See
+  *Supported field types* above.
 - **Reset fulltext search** (optional): adds an empty fulltext query parameter
   so following the link starts a fresh, facet-only search. The parameter name
   (default `search_api_fulltext`) is configurable to match your view's exposed

config/schema/solr_facet_field_formatter.schema.yml

@@ -8,6 +8,9 @@ field.formatter.settings.solr_facet_text_field_formatter:
     facet_id:
       type: string
       label: 'Facet'
+    value_source:
+      type: string
+      label: 'Facet value source'
     reset_fulltext:
       type: boolean
       label: 'Reset fulltext search'

src/Plugin/Field/FieldFormatter/SolrFacetTextFieldFormatter.php

@@ -88,6 +88,11 @@ class SolrFacetTextFieldFormatter extends FormatterBase implements ContainerFact
       'facet_source' => '',
       // The machine name of the facet to filter on.
       'facet_id' => '',
+      // Which value of the field to send to the facet: 'auto' (referenced
+      // entity label for reference fields, field value otherwise), 'label'
+      // (force the referenced entity label) or 'raw' (the stored value, i.e.
+      // the entity id for reference fields).
+      'value_source' => 'auto',
       // Whether to reset an exposed fulltext filter on the target page so the
       // link produces a clean, facet-only search.
       'reset_fulltext' => TRUE,
@@ -122,6 +127,14 @@ class SolrFacetTextFieldFormatter extends FormatterBase implements ContainerFact
       '#required' => TRUE,
     ];
 
+    $form['value_source'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Facet value'),
+      '#description' => $this->t('Which value of the field to filter the facet by. Only affects reference fields (entity reference, typed relation, …).'),
+      '#options' => $this->getValueSourceOptions(),
+      '#default_value' => $this->getSetting('value_source'),
+    ];
+
     $form['reset_fulltext'] = [
       '#type' => 'checkbox',
       '#title' => $this->t('Reset fulltext search'),
@@ -158,6 +171,13 @@ class SolrFacetTextFieldFormatter extends FormatterBase implements ContainerFact
         '%facet' => $this->getFacetLabel($facet_id),
         '%source' => $this->getFacetSourceLabel($facet_source),
       ]);
+      $value_source = $this->getSetting('value_source');
+      if ($value_source && $value_source !== 'auto') {
+        $options = $this->getValueSourceOptions();
+        if (isset($options[$value_source])) {
+          $summary[] = $this->t('Facet value: @value.', ['@value' => $options[$value_source]]);
+        }
+      }
     }
     else {
       $summary[] = $this->t('Not configured: select a facet source and facet.');
@@ -213,13 +233,31 @@ class SolrFacetTextFieldFormatter extends FormatterBase implements ContainerFact
     return $element;
   }
 
+  /**
+   * The available "Facet value" options for the settings form.
+   *
+   * @return array
+   *   An array of human readable labels keyed by value source id.
+   */
+  protected function getValueSourceOptions() {
+    return [
+      'auto' => $this->t('Automatic (referenced entity label, or the field value)'),
+      'label' => $this->t('Referenced entity label'),
+      'raw' => $this->t('Raw stored value (entity ID for reference fields)'),
+    ];
+  }
+
   /**
    * Extracts the value to filter the facet by from a field item.
    *
-   * For reference fields (entity_reference, typed_relation, …) the referenced
-   * entity's label is used, so the facet is filtered by the readable value
-   * rather than the internal entity id. For all other field types the field's
-   * main property value is used.
+   * The "Facet value" setting controls how reference fields (entity_reference,
+   * typed_relation, …) are resolved:
+   * - 'auto': the referenced entity's label, so the facet is filtered by the
+   *   readable value rather than the internal entity id.
+   * - 'label': force the referenced entity's label.
+   * - 'raw': the stored value (the entity id for reference fields).
+   *
+   * For non-reference fields the field's main property value is always used.
    *
    * @param \Drupal\Core\Field\FieldItemInterface $item
    *   The field item.
@@ -228,11 +266,15 @@ class SolrFacetTextFieldFormatter extends FormatterBase implements ContainerFact
    *   The facet value, or NULL if none could be determined.
    */
   protected function getFacetValue(FieldItemInterface $item) {
-    if ($item instanceof EntityReferenceItem) {
+    $value_source = $this->getSetting('value_source');
+
+    if ($value_source !== 'raw' && $item instanceof EntityReferenceItem) {
       $entity = $item->entity;
       if ($entity instanceof EntityInterface) {
         return $entity->label();
       }
+      // For an explicit 'label' request with no loadable entity, fall through
+      // to the raw value below rather than returning nothing.
     }
 
     $main_property = $item->getFieldDefinition()