#7102 Fix enhanced client list projection placeholders

.changes/next-release/bugfix-AmazonDynamoDBEnhancedClient-8a26648.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon DynamoDB Enhanced Client",
+    "description": "Fix projection expression placeholders for list-index attributes in the enhanced DynamoDB client. Resolves [#7102](https://github.com/aws/aws-sdk-java-v2/issues/7102).",
+    "contributor": "Arnab"
+}

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/ProjectionExpression.java

@@ -93,7 +93,8 @@
 public class ProjectionExpression {
 
     private static final String AMZN_MAPPED = "#AMZN_MAPPED_";
-    private static final UnaryOperator<String> PROJECTION_EXPRESSION_KEY_MAPPER = k -> AMZN_MAPPED + cleanAttributeName(k);
+    private static final UnaryOperator<String> PROJECTION_EXPRESSION_KEY_MAPPER =
+        k -> AMZN_MAPPED + cleanAttributeName(attributeNameWithoutListDereference(k));
 
     private final Optional<String> projectionExpressionAsString;
     private final Map<String, String> expressionAttributeNames;
@@ -132,6 +133,7 @@ private static Map<String, String> createAttributePlaceholders(List<NestedAttrib
         Map<String, List<String>> placeholderToAttributeNames =
             nestedAttributeNames.stream()
                                 .flatMap(n -> n.elements().stream())
+                                .map(ProjectionExpression::attributeNameWithoutListDereference)
                                 .distinct()
                                 .collect(Collectors.groupingBy(PROJECTION_EXPRESSION_KEY_MAPPER, Collectors.toList()));
 
@@ -180,8 +182,45 @@ private static String convertToNameExpression(NestedAttributeName nestedAttribut
                                                   Map<String, String> attributeToSanitizedMap) {
         return nestedAttributeName.elements()
                                   .stream()
-                                  .map(attributeToSanitizedMap::get)
+                                  .map(element -> attributeToSanitizedMap.get(attributeNameWithoutListDereference(element))
+                                                  + listDereferenceSuffix(element))
                                   .collect(Collectors.joining("."));
     }
 
+    private static String attributeNameWithoutListDereference(String attributeName) {
+        int firstListDereferenceIndex = firstListDereferenceIndex(attributeName);
+        return firstListDereferenceIndex == -1 ? attributeName : attributeName.substring(0, firstListDereferenceIndex);
+    }
+
+    private static String listDereferenceSuffix(String attributeName) {
+        int firstListDereferenceIndex = firstListDereferenceIndex(attributeName);
+        return firstListDereferenceIndex == -1 ? "" : attributeName.substring(firstListDereferenceIndex);
+    }
+
+    private static int firstListDereferenceIndex(String attributeName) {
+        int position = attributeName.length();
+        int firstListDereferenceIndex = position;
+
+        while (position > 0 && attributeName.charAt(position - 1) == ']') {
+            int openBracketIndex = attributeName.lastIndexOf('[', position - 1);
+            if (openBracketIndex == -1 || openBracketIndex == position - 1 ||
+                !containsOnlyDigits(attributeName, openBracketIndex + 1, position - 1)) {
+                return -1;
+            }
+            firstListDereferenceIndex = openBracketIndex;
+            position = openBracketIndex;
+        }
+
+        return firstListDereferenceIndex == attributeName.length() ? -1 : firstListDereferenceIndex;
+    }
+
+    private static boolean containsOnlyDigits(String value, int beginIndex, int endIndex) {
+        for (int i = beginIndex; i < endIndex; i++) {
+            if (!Character.isDigit(value.charAt(i))) {
+                return false;
+            }
+        }
+        return true;
+    }
+
 }

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/ProjectionExpressionTest.java

@@ -22,7 +22,7 @@ public class ProjectionExpressionTest {
     static {
         EXPECTED_ATTRIBUTE_NAMES.put("#AMZN_MAPPED_attribute", "attribute");
         EXPECTED_ATTRIBUTE_NAMES.put("#AMZN_MAPPED_Attribute", "Attribute");
-        EXPECTED_ATTRIBUTE_NAMES.put("#AMZN_MAPPED_firstiteminlist[0]", "firstiteminlist[0]");
+        EXPECTED_ATTRIBUTE_NAMES.put("#AMZN_MAPPED_firstiteminlist", "firstiteminlist");
         EXPECTED_ATTRIBUTE_NAMES.put("#AMZN_MAPPED_March_2021", "March-2021");
         EXPECTED_ATTRIBUTE_NAMES.put("#AMZN_MAPPED_Why_Make_This_An_Attribute_Name", "Why.Make-This*An:Attribute:Name");
     }
@@ -104,6 +104,23 @@ public void nonUniquePlaceholders_AreDisambiguated() {
         assertProjectionExpression(attributeNames, expectedAttributeNames, expectedProjectionExpression);
     }
 
+    @Test
+    public void listDereferenceAttributes_AreMappedToAttributeName() {
+        Map<String, String> expectedAttributeNames = new HashMap<>();
+        expectedAttributeNames.put("#AMZN_MAPPED_tags", "tags");
+        expectedAttributeNames.put("#AMZN_MAPPED_metadata", "metadata");
+        expectedAttributeNames.put("#AMZN_MAPPED_values", "values");
+
+        List<NestedAttributeName> attributeNames = Arrays.asList(
+            NestedAttributeName.create("tags[0]"),
+            NestedAttributeName.create("metadata", "values[1][2]")
+        );
+
+        String expectedProjectionExpression = "#AMZN_MAPPED_tags[0],#AMZN_MAPPED_metadata.#AMZN_MAPPED_values[1][2]";
+
+        assertProjectionExpression(attributeNames, expectedAttributeNames, expectedProjectionExpression);
+    }
+
     private void assertProjectionExpression(List<NestedAttributeName> attributeNames,
                                             Map<String, String> expectedAttributeNames,
                                             String expectedProjectionExpression) {

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/internal/operations/ScanOperationTest.java

@@ -229,6 +229,25 @@ public void generateRequest_projectionExpression() {
         assertThat(request, is(expectedRequest));
     }
 
+    @Test
+    public void generateRequest_projectionExpressionWithListDereference() {
+        ScanOperation<FakeItem> operation = ScanOperation.create(
+                ScanEnhancedRequest.builder()
+                        .addAttributeToProject("tags[0]")
+                        .build()
+        );
+        ScanRequest request = operation.generateRequest(FakeItem.getTableSchema(),
+                PRIMARY_CONTEXT,
+                null);
+
+        ScanRequest expectedRequest = ScanRequest.builder()
+                .tableName(TABLE_NAME)
+                .projectionExpression("#AMZN_MAPPED_tags[0]")
+                .expressionAttributeNames(singletonMap("#AMZN_MAPPED_tags", "tags"))
+                .build();
+        assertThat(request, is(expectedRequest));
+    }
+
     @Test
     public void generateRequest_hashKeyOnly_exclusiveStartKey() {
         FakeItem exclusiveStartKey = createUniqueFakeItem();