Feat(rerank): Add no-op DocumentPostProcessor as fallback

- Register a default DocumentPostProcessor that returns documents as-is

Signed-off-by: KoreaNirsa <[email protected]>

Author: KoreaNirsa

spring-ai-rag/src/main/java/org/springframework/ai/rag/postretrieval/rerank/RerankConfig.java

@@ -2,6 +2,7 @@
 
 import org.springframework.ai.rag.postretrieval.document.DocumentPostProcessor;
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -24,9 +25,32 @@ public class RerankConfig {
     @Value("${spring.ai.rerank.cohere.api-key}")
     private String apiKey;
 
+    /**
+     * Registers a DocumentPostProcessor bean that enables reranking using Cohere.
+     * 
+     * This bean is only created when the property `spring.ai.rerank.enabled=true` is set.
+     * The API key is injected from application properties or environment variables.
+     *
+     * @return An instance of RerankerPostProcessor backed by Cohere API
+     */
     @Bean
     @ConditionalOnProperty(name = "spring.ai.rerank.enabled", havingValue = "true")
     public DocumentPostProcessor rerankerPostProcessor() {
         return new RerankerPostProcessor(CohereApi.builder().apiKey(apiKey).build());
     }
+    
+    /**
+     * Provides a fallback DocumentPostProcessor when reranking is disabled
+     * or no custom implementation is registered.
+     *
+     * This implementation performs no reranking and simply returns the original list of documents.
+     * If additional post-processing is required, a custom bean should be defined.
+     *
+     * @return A pass-through DocumentPostProcessor that returns input as-is
+     */
+    @Bean
+    @ConditionalOnMissingBean
+    public DocumentPostProcessor noOpPostProcessor() {
+        return (query, documents) -> documents;
+    }
 }