Fix MongoDB index creation API compatibility across Spring Data MongoDB versions

Use reflection to handle API differences between Spring Data MongoDB versions:
- Spring Data MongoDB 4.2.x - 4.4.x: only ensureIndex() is available
- Spring Data MongoDB 4.5.x+: createIndex() is the new API, ensureIndex is deprecated

This fix ensures compatibility with both Spring Boot 3.4.x (MongoDB 4.4.x) and
Spring Boot 3.5.x (MongoDB 4.5.x+), which are both officially supported versions.

Fixes #4884

Author: JGoP-L

auto-configurations/models/chat/memory/repository/spring-ai-autoconfigure-model-chat-memory-repository-mongodb/src/main/java/org/springframework/ai/model/chat/memory/repository/mongo/autoconfigure/MongoChatMemoryIndexCreatorAutoConfiguration.java

@@ -16,6 +16,8 @@
 
 package org.springframework.ai.model.chat.memory.repository.mongo.autoconfigure;
 
+import java.lang.reflect.Method;
+
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -27,6 +29,8 @@
 import org.springframework.data.domain.Sort;
 import org.springframework.data.mongodb.core.MongoTemplate;
 import org.springframework.data.mongodb.core.index.Index;
+import org.springframework.data.mongodb.core.index.IndexDefinition;
+import org.springframework.data.mongodb.core.index.IndexOperations;
 
 /**
  * Class responsible for creating proper MongoDB indices for the ChatMemory. Creates a
@@ -57,26 +61,82 @@ public MongoChatMemoryIndexCreatorAutoConfiguration(MongoTemplate mongoTemplate,
 	public void initIndicesAfterStartup() {
 		logger.info("Creating MongoDB indices for ChatMemory");
 		// Create a main index
-		this.mongoTemplate.indexOps(Conversation.class)
-			.createIndex(new Index().on("conversationId", Sort.Direction.ASC).on("timestamp", Sort.Direction.DESC));
-
+		createMainIndex();
 		createOrUpdateTtlIndex();
 	}
 
+	private void createMainIndex() {
+		var indexOps = this.mongoTemplate.indexOps(Conversation.class);
+		var index = new Index().on("conversationId", Sort.Direction.ASC).on("timestamp", Sort.Direction.DESC);
+		
+		// Use reflection to handle API differences across Spring Data MongoDB versions
+		createIndexSafely(indexOps, index);
+	}
+
 	private void createOrUpdateTtlIndex() {
 		if (!this.mongoChatMemoryProperties.getTtl().isZero()) {
+			var indexOps = this.mongoTemplate.indexOps(Conversation.class);
 			// Check for existing TTL index
-			this.mongoTemplate.indexOps(Conversation.class).getIndexInfo().forEach(idx -> {
+			indexOps.getIndexInfo().forEach(idx -> {
 				if (idx.getExpireAfter().isPresent()
 						&& !idx.getExpireAfter().get().equals(this.mongoChatMemoryProperties.getTtl())) {
 					logger.warn("Dropping existing TTL index, because TTL is different");
-					this.mongoTemplate.indexOps(Conversation.class).dropIndex(idx.getName());
+					indexOps.dropIndex(idx.getName());
 				}
 			});
-			this.mongoTemplate.indexOps(Conversation.class)
-				.createIndex(new Index().on("timestamp", Sort.Direction.ASC)
+			// Use reflection to handle API differences across Spring Data MongoDB versions
+			createIndexSafely(indexOps, new Index().on("timestamp", Sort.Direction.ASC)
 					.expire(this.mongoChatMemoryProperties.getTtl()));
 		}
 	}
 
+	/**
+	 * Creates an index using reflection to handle API changes across different Spring
+	 * Data MongoDB versions:
+	 * <ul>
+	 * <li>Spring Data MongoDB 4.2.x - 4.4.x: only {@code ensureIndex(IndexDefinition)}
+	 * is available</li>
+	 * <li>Spring Data MongoDB 4.5.x+: {@code createIndex(IndexDefinition)} is the new
+	 * API, {@code ensureIndex} is deprecated</li>
+	 * </ul>
+	 * @param indexOps the IndexOperations instance
+	 * @param index the index definition
+	 * @throws IllegalStateException if neither method is available or invocation fails
+	 */
+	private void createIndexSafely(IndexOperations indexOps, IndexDefinition index) {
+		try {
+			// Try new API (Spring Data MongoDB 4.5.x+)
+			Method method = IndexOperations.class.getMethod("createIndex", IndexDefinition.class);
+			method.invoke(indexOps, index);
+			logger.debug("Created index using createIndex() method");
+			return;
+		}
+		catch (NoSuchMethodException createIndexNotFound) {
+			// Fall back to old API (Spring Data MongoDB 4.2.x - 4.4.x)
+			try {
+				Method method = IndexOperations.class.getMethod("ensureIndex", IndexDefinition.class);
+				method.invoke(indexOps, index);
+				logger.debug("Created index using ensureIndex() method");
+				return;
+			}
+			catch (NoSuchMethodException ensureIndexNotFound) {
+				// Neither method exists - this should never happen with supported versions
+				String message = "Neither createIndex() nor ensureIndex() method found on IndexOperations. "
+						+ "This may indicate an unsupported Spring Data MongoDB version.";
+				logger.error(message);
+				throw new IllegalStateException(message, ensureIndexNotFound);
+			}
+			catch (ReflectiveOperationException ex) {
+				String message = "Failed to invoke ensureIndex() method";
+				logger.error(message, ex);
+				throw new IllegalStateException(message, ex);
+			}
+		}
+		catch (ReflectiveOperationException ex) {
+			String message = "Failed to invoke createIndex() method";
+			logger.error(message, ex);
+			throw new IllegalStateException(message, ex);
+		}
+	}
+
 }