Styjun

Currently, I'm using Spring boot with DynamoDB and using DynamoDBMapper for all DB operation. Right now it's sorting in ascending order based on sort key(field name id). How can I sort in descending order based on sort key value?

Current code :

@Override
public PageImpl<Order> getCustomerOrders(Pageable pageable) 

 List<Order> orders = new ArrayList<>();


 Map<String, AttributeValue> eav = new HashMap<String, AttributeValue>();


 DynamoDBScanExpression scanExpression = new DynamoDBScanExpression();
 List<String> exp = new ArrayList<>();


 // filters

 if (!exp.isEmpty())
 scanExpression
 .withFilterExpression(String.join(" AND ", exp)).withExpressionAttributeValues(eav);


 PaginatedScanList<Order> scan = mapper.scan(Order.class, scanExpression);




 int start = (int) pageable.getOffset();
 int size = scan.size();
 int end = (start + pageable.getPageSize()) > size ? size : (start + pageable.getPageSize());

 if (start < size)
 for (Order o : scan.subList(start, end)) 
 orders.add(o);
 

 return new PageImpl<Order>(orders, pageable, size);

After some googling, I found that I need to set ScanIndexForward to be true but still I'm confused that how can I achieve it with DynamoDBMApper?

Or should I need to use any low-level class like DynamoDB or AmazonDynamoDb?

Finally, I found the solution myself, Thanks to @ydrall for the hint. As per his comment, ScanIndexForward can only be used with query option so I just updated with query. I had created DynamoDBQueryExpression object and updated the property using setScanIndexForward method ( false ).

Full java code :

@Override
public PageImpl<Order> getAllProgramOrder(Pageable pageable, OrderFilter filter) 
 List<Order> orders = new ArrayList<>();


 DynamoDBQueryExpression<Order> queryExpression = new DynamoDBQueryExpression<Order>();

 queryExpression.setScanIndexForward(false);


 Map<String, AttributeValue> eav = new HashMap<String, AttributeValue>();
 eav.put(":val1", new AttributeValue().withS(authenticationFacade.getProgramId()));

 queryExpression.withKeyConditionExpression("hashKey = :val1 ")
 .withExpressionAttributeValues(eav);

 PaginatedQueryList<Order> scan = mapper.query(Order.class, queryExpression);


 int start = (int) pageable.getOffset();
 int size = scan.size();
 int end = (start + pageable.getPageSize()) > size ? size : (start + pageable.getPageSize());

 if (start < size)
 for (Order o : scan.subList(start, end)) 
 orders.add(o);
 

 return new PageImpl<Order>(orders, pageable, size);

Ref : How can I Scan an index in reverse in DynamoDB?