feat: refine personalization query by merging with user context

- Merge initial query with user context to create a comprehensive prompt
- Only skip reranking if both query and user_context are None
- Support reranking with query-only, user_context-only, or both
- Use 'let else' pattern for cleaner error handling
- Add comprehensive tests for different parameter combinations
- Improve prompt format for better reranking effectiveness

crates/meilisearch/src/personalization/mod.rs

@@ -29,10 +29,19 @@ impl PersonalizationService {
         personalize: Option<&Personalize>,
         query: Option<&str>,
     ) -> Result<SearchResult, ResponseError> {
-        // If personalization is not requested, no API key, or no query, return original results
-        let Some(_personalize) = personalize else { return Ok(search_result) };
+        // If no API key, return original results
         let Some(cohere) = &self.cohere else { return Ok(search_result) };
-        let Some(query) = query else { return Ok(search_result) };
+
+        // Extract user context from personalization
+        let user_context = personalize.and_then(|p| p.user_context.as_deref());
+
+        // Build the prompt by merging query and user context
+        let prompt = match (query, user_context) {
+            (Some(q), Some(uc)) => format!("User Context: {}\nQuery: {}", uc, q),
+            (Some(q), None) => q.to_string(),
+            (None, Some(uc)) => format!("User Context: {}", uc),
+            (None, None) => return Ok(search_result),
+        };
 
         // Extract documents for reranking
         let documents: Vec<String> = search_result
@@ -50,7 +59,7 @@ impl PersonalizationService {
 
         // Prepare the rerank request
         let rerank_request = ReRankRequest {
-            query,
+            query: &prompt,
             documents: &documents,
             model: ReRankModel::EnglishV3, // Use the default and more recent model
             top_n: None,
@@ -124,7 +133,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn test_personalization_service_disabled() {
+    async fn test_personalization_service_with_user_context_only() {
         let service = PersonalizationService::new(Some("fake_key".to_string()));
         let personalize = Personalize { user_context: Some("test user".to_string()) };
 
@@ -146,12 +155,71 @@ mod tests {
             used_negative_operator: false,
         };
 
-        let result = service
-            .rerank_search_results(search_result.clone(), Some(&personalize), Some("test"))
-            .await;
+        let result =
+            service.rerank_search_results(search_result.clone(), Some(&personalize), None).await;
         assert!(result.is_ok());
 
-        // Should return original results when personalization is disabled
+        // Should attempt reranking with user context only
+        let reranked_result = result.unwrap();
+        assert_eq!(reranked_result.hits.len(), search_result.hits.len());
+    }
+
+    #[tokio::test]
+    async fn test_personalization_service_with_query_only() {
+        let service = PersonalizationService::new(Some("fake_key".to_string()));
+
+        let search_result = SearchResult {
+            hits: vec![SearchHit {
+                document: serde_json::Map::new(),
+                formatted: serde_json::Map::new(),
+                matches_position: None,
+                ranking_score: Some(1.0),
+                ranking_score_details: None,
+            }],
+            query: "test".to_string(),
+            processing_time_ms: 10,
+            hits_info: HitsInfo::OffsetLimit { limit: 1, offset: 0, estimated_total_hits: 1 },
+            facet_distribution: None,
+            facet_stats: None,
+            semantic_hit_count: None,
+            degraded: false,
+            used_negative_operator: false,
+        };
+
+        let result = service.rerank_search_results(search_result.clone(), None, Some("test")).await;
+        assert!(result.is_ok());
+
+        // Should attempt reranking with query only
+        let reranked_result = result.unwrap();
+        assert_eq!(reranked_result.hits.len(), search_result.hits.len());
+    }
+
+    #[tokio::test]
+    async fn test_personalization_service_both_none() {
+        let service = PersonalizationService::new(Some("fake_key".to_string()));
+
+        let search_result = SearchResult {
+            hits: vec![SearchHit {
+                document: serde_json::Map::new(),
+                formatted: serde_json::Map::new(),
+                matches_position: None,
+                ranking_score: Some(1.0),
+                ranking_score_details: None,
+            }],
+            query: "test".to_string(),
+            processing_time_ms: 10,
+            hits_info: HitsInfo::OffsetLimit { limit: 1, offset: 0, estimated_total_hits: 1 },
+            facet_distribution: None,
+            facet_stats: None,
+            semantic_hit_count: None,
+            degraded: false,
+            used_negative_operator: false,
+        };
+
+        let result = service.rerank_search_results(search_result.clone(), None, None).await;
+        assert!(result.is_ok());
+
+        // Should return original results when both query and user_context are None
         let reranked_result = result.unwrap();
         assert_eq!(reranked_result.hits.len(), search_result.hits.len());
     }