Use retrievers in search query (#1835)

* Use retrievers in search query

* Use boolean query DSL (#1851)

* Update src/api/Elastic.Documentation.Api.Infrastructure/Adapters/Search/ElasticsearchGateway.cs

---------

Co-authored-by: Martijn Laarman <Mpdreamz@gmail.com>

Author: reakaleek

src/api/Elastic.Documentation.Api.Infrastructure/Adapters/Search/ElasticsearchGateway.cs

@@ -67,7 +67,66 @@ public ElasticsearchGateway(ElasticsearchOptions elasticsearchOptions, ILogger<E
 	}
 
 	public async Task<(int TotalHits, List<SearchResultItem> Results)> SearchAsync(string query, int pageNumber, int pageSize, Cancel ctx = default) =>
-		await ExactSearchAsync(query, pageNumber, pageSize, ctx);
+		await HybridSearchWithRrfAsync(query, pageNumber, pageSize, ctx);
+
+	public async Task<(int TotalHits, List<SearchResultItem> Results)> HybridSearchWithRrfAsync(string query, int pageNumber, int pageSize, Cancel ctx = default)
+	{
+		_logger.LogInformation("Starting RRF hybrid search for '{Query}' with pageNumber={PageNumber}, pageSize={PageSize}", query, pageNumber, pageSize);
+
+		var searchQuery = query.Replace("dotnet", "net", StringComparison.InvariantCultureIgnoreCase);
+
+		var lexicalSearchRetriever =
+			((Query)new PrefixQuery(Infer.Field<DocumentDto>(f => f.Title.Suffix("keyword")), searchQuery) { Boost = 10.0f, CaseInsensitive = true }
+				|| new MatchQuery(Infer.Field<DocumentDto>(f => f.Title), searchQuery) { Operator = Operator.And, Boost = 8.0f }
+				|| new MatchBoolPrefixQuery(Infer.Field<DocumentDto>(f => f.Title), searchQuery) { Boost = 6.0f }
+				|| new MatchQuery(Infer.Field<DocumentDto>(f => f.Abstract), searchQuery) { Boost = 4.0f }
+				|| new MatchQuery(Infer.Field<DocumentDto>(f => f.Parents.First().Title), searchQuery) { Boost = 2.0f }
+				|| new MatchQuery(Infer.Field<DocumentDto>(f => f.Title), searchQuery) { Fuzziness = 1, Boost = 1.0f }
+			)
+				&& !(Query)new TermsQuery(Infer.Field<DocumentDto>(f => f.Url.Suffix("keyword")), new TermsQueryField(["/docs", "/docs/", "/docs/404", "/docs/404/"]))
+			;
+		var semanticSearchRetriever =
+				((Query)new SemanticQuery("title.semantic_text", searchQuery) { Boost = 5.0f }
+				 || new SemanticQuery("abstract", searchQuery) { Boost = 3.0f }
+				)
+				&& !(Query)new TermsQuery(Infer.Field<DocumentDto>(f => f.Url.Suffix("keyword")),
+					new TermsQueryField(["/docs", "/docs/", "/docs/404", "/docs/404/"]))
+			;
+
+		try
+		{
+			var response = await _client.SearchAsync<DocumentDto>(s => s
+				.Indices(_elasticsearchOptions.IndexName)
+				.Retriever(r => r
+					.Rrf(rrf => rrf
+						.Retrievers(
+							// Lexical/Traditional search retriever
+							ret => ret.Standard(std => std.Query(lexicalSearchRetriever)),
+							// Semantic search retriever
+							ret => ret.Standard(std => std.Query(semanticSearchRetriever))
+						)
+						.RankConstant(60) // Controls how much weight is given to document ranking
+					)
+				)
+				.From((pageNumber - 1) * pageSize)
+				.Size(pageSize), ctx);
+
+			if (!response.IsValidResponse)
+			{
+				_logger.LogWarning("Elasticsearch RRF search response was not valid. Reason: {Reason}",
+					response.ElasticsearchServerError?.Error?.Reason ?? "Unknown");
+			}
+			else
+				_logger.LogInformation("RRF search completed for '{Query}'. Total hits: {TotalHits}", query, response.Total);
+
+			return ProcessSearchResponse(response);
+		}
+		catch (Exception ex)
+		{
+			_logger.LogError(ex, "Error occurred during Elasticsearch RRF search for '{Query}'", query);
+			throw;
+		}
+	}
 
 	public async Task<(int TotalHits, List<SearchResultItem> Results)> ExactSearchAsync(string query, int pageNumber, int pageSize, Cancel ctx = default)
 	{