Return facets even when there is no value associated to it

milli/src/search/facet/facet_distribution.rs

@@ -243,9 +243,7 @@ impl<'a> FacetDistribution<'a> {
         for (fid, name) in fields_ids_map.iter() {
             if crate::is_faceted(name, &fields) {
                 let values = self.facet_values(fid)?;
-                if !values.is_empty() {
-                    distribution.insert(name.to_string(), values);
-                }
+                distribution.insert(name.to_string(), values);
             }
         }
 

milli/tests/search/facet_distribution.rs

@@ -0,0 +1,77 @@
+use std::io::Cursor;
+
+use big_s::S;
+use heed::EnvOpenOptions;
+use maplit::hashset;
+use milli::documents::{DocumentBatchBuilder, DocumentBatchReader};
+use milli::update::{IndexDocuments, IndexDocumentsConfig, IndexerConfig, Settings};
+use milli::{FacetDistribution, Index};
+
+#[test]
+fn test_facet_distribution_with_no_facet_values() {
+    let path = tempfile::tempdir().unwrap();
+    let mut options = EnvOpenOptions::new();
+    options.map_size(10 * 1024 * 1024); // 10 MB
+    let index = Index::new(options, &path).unwrap();
+
+    let mut wtxn = index.write_txn().unwrap();
+    let config = IndexerConfig::default();
+    let mut builder = Settings::new(&mut wtxn, &index, &config);
+
+    builder.set_filterable_fields(hashset! {
+        S("genres"),
+        S("tags"),
+    });
+    builder.execute(|_| ()).unwrap();
+
+    // index documents
+    let config = IndexerConfig { max_memory: Some(10 * 1024 * 1024), ..Default::default() };
+    let indexing_config = IndexDocumentsConfig { autogenerate_docids: true, ..Default::default() };
+
+    let mut builder =
+        IndexDocuments::new(&mut wtxn, &index, &config, indexing_config, |_| ()).unwrap();
+    let mut cursor = Cursor::new(Vec::new());
+    let mut documents_builder = DocumentBatchBuilder::new(&mut cursor).unwrap();
+    let reader = Cursor::new(
+        r#"[
+        {
+            "id": 123,
+            "title": "What a week, hu...",
+            "genres": [],
+            "tags": ["blue"]
+        },
+        {
+            "id": 345,
+            "title": "I am the pig!",
+            "tags": ["red"]
+        }
+    ]"#,
+    );
+
+    for doc in serde_json::Deserializer::from_reader(reader).into_iter::<serde_json::Value>() {
+        let doc = Cursor::new(serde_json::to_vec(&doc.unwrap()).unwrap());
+        documents_builder.extend_from_json(doc).unwrap();
+    }
+
+    documents_builder.finish().unwrap();
+
+    cursor.set_position(0);
+
+    // index documents
+    let content = DocumentBatchReader::from_reader(cursor).unwrap();
+    builder.add_documents(content).unwrap();
+    builder.execute().unwrap();
+
+    wtxn.commit().unwrap();
+
+    let txn = index.read_txn().unwrap();
+    let mut distrib = FacetDistribution::new(&txn, &index);
+    distrib.facets(vec!["genres"]);
+    let result = distrib.execute().unwrap();
+    assert_eq!(result["genres"].len(), 0);
+
+    let mut distrib = FacetDistribution::new(&txn, &index);
+    distrib.facets(vec!["tags"]);
+    let result = distrib.execute().unwrap();
+    assert_eq!(result["tags"].len(), 2);
+}

milli/tests/search/mod.rs

@@ -13,6 +13,7 @@ use serde::Deserialize;
 use slice_group_by::GroupBy;
 
 mod distinct;
+mod facet_distribution;
 mod filters;
 mod query_criteria;
 mod sort;