Fix VectorStoreDocumentRetriever to handle Filter.Expression objects directly

- Updated computeRequestFilterExpression to check if the context value is already
  a Filter.Expression object before attempting to parse it as a string
- Added docs for FILTER_EXPRESSION key that it accepts both String and Filter.Expression
- Added test

Fixes #3179

spring-ai-rag/src/main/java/org/springframework/ai/rag/retrieval/search/VectorStoreDocumentRetriever.java

@@ -45,6 +45,11 @@ import org.springframework.util.StringUtils;
  * List<Document> documents = retriever.retrieve(new Query("example query"));
  * }</pre>
  *
+ * <p>
+ * The {@link #FILTER_EXPRESSION} context key can be used to provide a filter expression
+ * for a specific query. This key accepts either a string representation of a filter
+ * expression or a {@link Filter.Expression} object directly.
+ *
  * @author Thomas Vitale
  * @since 1.0.0
  */
@@ -89,10 +94,27 @@ public final class VectorStoreDocumentRetriever implements DocumentRetriever {
 		return this.vectorStore.similaritySearch(searchRequest);
 	}
 
+	/**
+	 * Computes the filter expression to use for the current request.
+	 * <p>
+	 * The filter expression can be provided in the query context using the
+	 * {@link #FILTER_EXPRESSION} key. This key accepts either a string representation of
+	 * a filter expression or a {@link Filter.Expression} object directly.
+	 * <p>
+	 * If no filter expression is provided in the context, the default filter expression
+	 * configured for this retriever is used.
+	 * @param query the query containing potential context with filter expression
+	 * @return the filter expression to use for the request
+	 */
 	private Filter.Expression computeRequestFilterExpression(Query query) {
 		var contextFilterExpression = query.context().get(FILTER_EXPRESSION);
-		if (contextFilterExpression != null && StringUtils.hasText(contextFilterExpression.toString())) {
-			return new FilterExpressionTextParser().parse(contextFilterExpression.toString());
+		if (contextFilterExpression != null) {
+			if (contextFilterExpression instanceof Filter.Expression) {
+				return (Filter.Expression) contextFilterExpression;
+			}
+			else if (StringUtils.hasText(contextFilterExpression.toString())) {
+				return new FilterExpressionTextParser().parse(contextFilterExpression.toString());
+			}
 		}
 		return this.filterExpression.get();
 	}

spring-ai-rag/src/test/java/org/springframework/ai/rag/retrieval/search/VectorStoreDocumentRetrieverTests.java

@@ -234,6 +234,32 @@ class VectorStoreDocumentRetrieverTests {
 			.isEqualTo(new FilterExpressionBuilder().eq("location", "Rivendell").build());
 	}
 
+	@Test
+	void retrieveWithQueryObjectAndFilterExpressionObject() {
+		var mockVectorStore = mock(VectorStore.class);
+		var documentRetriever = VectorStoreDocumentRetriever.builder().vectorStore(mockVectorStore).build();
+
+		// Create a Filter.Expression object directly
+		var filterExpression = new Filter.Expression(EQ, new Filter.Key("location"), new Filter.Value("Rivendell"));
+
+		var query = Query.builder()
+			.text("test query")
+			.context(Map.of(VectorStoreDocumentRetriever.FILTER_EXPRESSION, filterExpression))
+			.build();
+		documentRetriever.retrieve(query);
+
+		// Verify the mock interaction
+		var searchRequestCaptor = ArgumentCaptor.forClass(SearchRequest.class);
+		verify(mockVectorStore).similaritySearch(searchRequestCaptor.capture());
+
+		// Verify the search request
+		var searchRequest = searchRequestCaptor.getValue();
+		assertThat(searchRequest.getQuery()).isEqualTo("test query");
+		assertThat(searchRequest.getSimilarityThreshold()).isEqualTo(SearchRequest.SIMILARITY_THRESHOLD_ACCEPT_ALL);
+		assertThat(searchRequest.getTopK()).isEqualTo(SearchRequest.DEFAULT_TOP_K);
+		assertThat(searchRequest.getFilterExpression()).isEqualTo(filterExpression);
+	}
+
 	static final class TenantContextHolder {
 
 		private static final ThreadLocal<String> tenantIdentifier = new ThreadLocal<>();