DynamoDBMapper: How to get saved item?

Question

For a simple Java REST-API I created a save function to persist my model to a DynamoDB table.

The model uses a auto generated range key as you can see here:

@DynamoDBTable(tableName = "Events")
public class EventModel {
    private int country;
    private String id;
    // ...

    @DynamoDBHashKey
    public int getCountry() {
        return country;
    }
    public void setCountry(int country) {
        this.country = country;
    }

    @DynamoDBRangeKey
    @DynamoDBAutoGeneratedKey
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }

    //...
}

Unfortunately the the DynamoDBMappers .save() method does not return anything. I want to return the created item to set the proper location header in my 201 HTTP response.

public EventModel create(EventModel event) {
    mapper.save(event);
    return null;
}

How can I make that work? Any suggestions? Of course I could generate the id on the client but I don´t want to do this because solving the potential atomicity issue needs additional logic on client- and server-side.

I´m using the aws-java-sdk-dynamodb in version 1.11.86.

score 10 · Accepted Answer · answered Jan 31 '17 at 17:02

10

Never mind, I figured out how to do it. The .save() method updates the reference of the object. After calling mapper.save(event); the id property is populated and has its value.

So the way to get it work is just:

public EventModel create(EventModel event) {
    mapper.save(event);
    return event;
}

That´s it!

answered Jan 31 '17 at 17:02

maxarndt

560
7
16

the event that is returned is not the one that is in the database though. – BinaryShrub Jun 17 '17 at 06:58
Mapper updates the content of the passed object. You will get back with id. – Cihangir Nov 11 '20 at 12:44
works for me, thanks, anyone know whats the magic behind it – Ping Woo Jun 23 '22 at 00:27

score 1 · Answer 2 · edited Dec 30 '19 at 08:44

There is direct way through dynamo db mapper to get what is saved in dynamodb after put/update Approach mentioned by m4xy@ would work if you are saving with DynamoDBConfig as CLOBBER or UPDATE. If you are using UPDATE_SKIP_NULL_ATTRIBUTES, this approach won't work. If you are using mapper, you have to specifically call db again to get existing value (which might have been updated if there are multiple writers and you might get unxpected result). To ensure read that you expect you can implement locking for write such that if lock is acquired by a given thread, no other thread can write for a given key. But, this approach as a downside of slowing down your application. Alternatively, you can use dynamoDBClient that has apis to support return db values after write. https://sdk.amazonaws.com/java/api/2.0.0-preview-11/index.html?software/amazon/awssdk/services/dynamodb/DynamoDbClient.html

DynamoDBMapper: How to get saved item?

2 Answers2