Aws Projectionexpression with Dynamodbscanexpression
I Have a Dynamodb Table (Named Student) with 17 Columns. in This Table I Have Columns Like Student_Id, Name, Age. .. . I Want to Get Students with Age > 18...
I have a dynamodb table (named Student) with 17 columns. In this table I have columns like student_id, name, age ... . I want to get students with age > 18. This code snippet is giving List<Student>. But I want only List<String> i.e list containing only student_id's.
DynamoDBScanExpression scanExpression = new DynamoDBScanExpression();
scanExpression.addFilterCondition("age", new Condition()
.withComparisonOperator(ComparisonOperator.GT)
.withAttributeValueList(new AttributeValue().withN(age)));
I tried this: scanExpression.setProjectionExpression("student_id");
But it throws error. So what wrong am I doing here. And if there is any way to do this task with DynamoDBQueryExpression Also if I want multiple selective cols, suggest a method for the same
1 Answer
Your code sample is not enough for me to see how you execute the scan expression. Is it using a DynamoDBMapper, if so the result of the scan always returns List of Objects of the class to which the table is mapped to. Providing a projection expression will only limit the attributes returned (Refer to this). You still have to extract the student_id into a list after the scan result is returned.
List<Student> result = mapper.scan(Student.class, scanExpression);
List<String> studentIds = result
.stream()
.map(student -> student.getStudentId())
.collect(Collectors.toList());
For more examples on Query and Scan refer to this AWS Documentation.
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