GH-3181: fix: Validate index name exists in CassandraVectorStore

Fixes: #3181

https://github.com/spring-projects/spring-ai/issues/3181

When using CassandraVectorStore with an invalid index name and
initializeSchema=false, the application would throw a generic
NoSuchElementException from Optional.get() with message "No value present"
when trying to access the index metadata.

Changes:
- Added explicit validation of index existence in checkSchemaValid() method
- Improved error handling in getIndexSimilarity() method to provide a clear
  error message when an index doesn't exist
- Added integration test to verify proper error handling with invalid index names

Signed-off-by: Soby Chacko <soby.chacko@broadcom.com>

vector-stores/spring-ai-cassandra-store/src/main/java/org/springframework/ai/vectorstore/cassandra/CassandraVectorStore.java

@@ -43,6 +43,7 @@ import com.datastax.oss.driver.api.core.cql.Row;
 import com.datastax.oss.driver.api.core.cql.SimpleStatement;
 import com.datastax.oss.driver.api.core.data.CqlVector;
 import com.datastax.oss.driver.api.core.metadata.schema.ColumnMetadata;
+import com.datastax.oss.driver.api.core.metadata.schema.IndexMetadata;
 import com.datastax.oss.driver.api.core.metadata.schema.TableMetadata;
 import com.datastax.oss.driver.api.core.type.DataType;
 import com.datastax.oss.driver.api.core.type.DataTypes;
@@ -397,11 +398,16 @@ public class CassandraVectorStore extends AbstractObservationVectorStore impleme
 
 	private Similarity getIndexSimilarity(TableMetadata metadata) {
 
-		return Similarity.valueOf(metadata.getIndex(this.schema.index())
-			.get()
-			.getOptions()
-			.getOrDefault("similarity_function", "COSINE")
-			.toUpperCase());
+		Optional<IndexMetadata> indexMetadata = metadata.getIndex(this.schema.index());
+
+		if (indexMetadata.isEmpty()) {
+			throw new IllegalStateException(
+					String.format("Index %s does not exist in table %s", this.schema.index(), this.schema.table));
+		}
+
+		return Similarity
+			.valueOf(indexMetadata.get().getOptions().getOrDefault("similarity_function", "COSINE").toUpperCase());
+
 	}
 
 	private PreparedStatement prepareDeleteStatement() {
@@ -554,6 +560,9 @@ public class CassandraVectorStore extends AbstractObservationVectorStore impleme
 			.getTable(this.schema.table)
 			.get();
 
+		Preconditions.checkState(tableMetadata.getIndex(this.schema.index()).isPresent(), "index %s does not exist",
+				this.schema.index());
+
 		Preconditions.checkState(tableMetadata.getColumn(this.schema.content).isPresent(), "column %s does not exist",
 				this.schema.content);
 

vector-stores/spring-ai-cassandra-store/src/test/java/org/springframework/ai/vectorstore/cassandra/CassandraVectorStoreIT.java

@@ -522,6 +522,31 @@ class CassandraVectorStoreIT extends BaseVectorStoreTests {
 		});
 	}
 
+	@Test
+	void throwsExceptionOnInvalidIndexNameWithSchemaValidation() {
+		this.contextRunner.run(context -> {
+			// Create valid schema first, then close
+			try (CassandraVectorStore validStore = createTestStore(context, new SchemaColumn("meta1", DataTypes.TEXT),
+					new SchemaColumn("meta2", DataTypes.TEXT))) {
+				// Nothing to do here. This should not fail as the Schema now exists
+			}
+
+			// Now try with invalid index name but don't reinitialize schema
+			CassandraVectorStore.Builder invalidBuilder = storeBuilder(context.getBean(CqlSession.class),
+					context.getBean(EmbeddingModel.class))
+				.addMetadataColumns(new SchemaColumn("meta1", DataTypes.TEXT),
+						new SchemaColumn("meta2", DataTypes.TEXT))
+				.indexName("non_existent_index_name")
+				.initializeSchema(false);
+
+			IllegalStateException exception = Assertions.assertThrows(IllegalStateException.class,
+					invalidBuilder::build);
+
+			assertThat(exception.getMessage()).contains("non_existent_index_name");
+			assertThat(exception.getMessage()).contains("does not exist");
+		});
+	}
+
 	@SpringBootConfiguration
 	@EnableAutoConfiguration(exclude = { DataSourceAutoConfiguration.class })
 	public static class TestApplication {