Support for inheritance in DynamoDbMapper

[sowdri]

I'm using dynamodb and the java class I'm storing has the following structure:

@DynamoDBTable(tableName="Animals")
class abstract Animal {

  @DynamoDBHashKey(attributeName = "Id")
  @DynamoDBAutoGeneratedKey
  String id;

}

class Dog extends Animal {
  // ... dog props
}

class Cat extends Animal {
  // ... cat props
}

I would like to save instances of Animal class to the db and retrieve them as well. Ready to supply meta information similar to @JsonTypeInfo and the likes.

Looks like this is not supported right now.

http://stackoverflow.com/questions/39221525/dynamodbmapper-support-for-inheritance

[fpoolev]

I was able to inherit from an abstract class using DynamoDBMapper by notating both the abstract class and the subclass with the @DynamoDBTable annotation.

[sowdri]

It would work if the operations are performed on Dog/Cat (concrete instances) vs Animal (abstract instance)

Support for polymorphism would mean operating on base class (put and get 'Animal' and the type information is handled automatically)

[fpoolev]

@sowdri Yeah, I only extended my abstract class as a means to add common attributes. I only attempted performing operations on my concrete classes. I did not attempt any polymorphism.

[kiiadi]

@sowdri - it's certain possible to save polymorphic types - in this case the mapper can get to the concrete implementation and uses the annotations on that to do the persistence to dynamo (ie: Cat will pick up additional properties of the Cat object and add those fields)

class Cat extends Animal {
  @DynamoDBAttribute(attributeName = "furLength")
  public String getFurLength()  { return furLength; }
  public void setFurLength()  { this.furLength = furLength; }
}

Animal cat = new Cat();
cat.setFurLength("long");
mapper.save(cat);

In this case, the furLength property will get persisted.

However at present the way the load mechanism works one must specify the concrete type you're trying to load.

I like the idea of having an additional annotation on the base-class that specifies sub-types to convert to - this would mean we'd need to store additional fields against the object in dynamo - but it could be a cool feature.

This is something we can look to add in a future release.

[sowdri]

@kiiadi Thanks for your reply.

For anyone who is looking for a solution for this problem. You could simple use Jackson for serialization/deserialization and use the low-level Item API (using the Item.fromJson() and Item.toJson()) for putting and getting polymorphic types in and out of Dynamo.

For anyone looking for a quick solution until this feature is implemented, I've posted the code of a base class just for reference.

public abstract class DynamoServiceImpl<T extends Base<String>> implements DynamoService<T> {

    @Autowired
    @AwsConfig.DynamoMapper
    private ObjectMapper objectMapper;

    @Autowired
    private DynamoDB db;

    abstract String getTable();

    abstract Class<?> getType();

    @Override
    public T get(String id) {
        Table table = db.getTable(getTable());
        Item item = table.getItem("id", id);
        String s = item.toJSON();

        Object obj;

        try {
            obj = objectMapper.readValue(s, getType());
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }

        return (T)obj;
    }

    @Override
    public T put(T obj) {
        /**
         * Check if item has id, else populate it
         */
        if (obj.getId() == null) {
            obj.setId(uuid());
        }

        Table table = db.getTable(getTable());

        String jsonStr;
        try {
            jsonStr = objectMapper.writeValueAsString(obj);
            System.out.println(jsonStr);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
            throw new RuntimeException("Cannot write item");
        }

        Item item = Item.fromJSON(jsonStr);

        table.putItem(item);
        return obj;
    }
}```

[ranjitparva]

+1 Looking for the exact same feature in DynamoDBMapper.

[arnoldtempfli]

+1

[matelang]

+1

[destebanm]

+1

[oscartoledo]

+1

[safrain]

+1

[bita-c]

+1