How to design a table in dynamodb for Carts - amazon-dynamodb

Question

I have a use case where in a person can have no. of carts. This is possible because 1) user can complete an order with a cart and after that cart is considered closed 2) user can leave a cart for 2 months and it is considered expired. If user adds new item on 2 months old cart, old cart is marked expired and new cart is generated.
I tried designing following table:
Table Name: Cart
Primary Hash Key: cartId (String)
Primary Range Key: updated (String)
I am using updated as a range column so that when I query it I can get all the carts sorted on when user updated those and I can pick the first (or last) one without sorting myself to have the most recent cart. However this is messing up my use cases.
If a user adds another item, I update the item in the cart and update the updated column as well. However this creates another cart for me (with same cart id but with new updated column). After re-reading the docs, I understand that primary key is composite of cartId and updated so probably I should remove it. However I believe my use case is genuine enough and it is bad that in my case I have to do sorting in application. Another way around would be to use an auto increment as range but that is non intuitive way of putting columns. If there is a work around pls let me know. I am using DynamoDBMapper and posting my classes (with only few fields).
import java.util.Set;
import com.amazonaws.services.dynamodbv2.datamodeling.*;
#DynamoDBTable(tableName="Cart")
public class Cart {
private String cartId;
private String email;
private Set<String> cartItemsJson;
private String status;
private String created;
private String updated;
#DynamoDBHashKey(attributeName="cartId")
public String getCartId() {
return cartId;
}
public void setCartId(String cartId) {
this.cartId = cartId;
}
#DynamoDBAttribute(attributeName="email")
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
#DynamoDBAttribute(attributeName="cartItems")
public Set<String> getCartItemsJson() {
return cartItemsJson;
}
public void setCartItemsJson(Set<String> cartItemsJson) {
this.cartItemsJson = cartItemsJson;
}
#DynamoDBAttribute(attributeName="status")
public String getStatus() {
return status;
}
public void setStatus(String status) {
this.status = status;
}
#DynamoDBAttribute(attributeName="created")
public String getCreated() {
return created;
}
public void setCreated(String created) {
this.created = created;
}
#DynamoDBRangeKey(attributeName="updated")
#DynamoDBVersionAttribute(attributeName="updated")
public String getUpdated() {
return updated;
}
public void setUpdated(String updated) {
this.updated = updated;
}
}
This the persistence layer code. I have tried various combinations of Save behaviour but still same results.
protected static DynamoDBMapper mapper = new DynamoDBMapper(dynamoDbClient);
mapper.save(cart,new DynamoDBMapperConfig(DynamoDBMapperConfig.SaveBehavior.UPDATE));

Answer 1

In DynamoDB you can not update the Hash or Range key. Updating them means deleting and create a new entry.
I know that you can create a Secondary index. Maybe this will help.
Also I think you can overcome the updated as range key. You can create the table as follows:
HashKey = userId
RangeKey = cardId ( but cardId needs to be sortable for each user )
normal column = updated
normal column = etc..
When you need the last cardId of a specific user, you can get top 1 rows for a hashkey=your user and reverse sorted so you get the last one first.
When you need to add an item to the card, you don't need to update the hash/range keys.
Hope it helps.