How to put a new key-value pair into a map in DynamoDB? (java)

22
votes

I have a DynamoDB attribute whose value is a map from a Number to a String. I am trying to put in a new key-value pair. From what I've read, this seems to be possible, but I don't know how.

I assume the solution is similar to the one in the link below:

How to update a Map or a List on AWS DynamoDB document API?

But I do not believe the example is on putting in a new item to a map. Could someone show me how to put in an item to a map?

Thanks.

EDIT:

I do not want to get the item, locally make the changes, and put it back. I am working with multiple clients who might interact concurrently (and I assume the update by dynamo ensures there will be no race conditions).

2
javaamazon-dynamodb
Keep in mind that DynamoDB is "eventually consistent". As in, even if you make an update to DynamoDB and then read it immediately, there's no guarantee that the update will be visible. This is more obvious when using replica tables. So avoiding the effects of pulling down, editing and writing back again won't really happen with doing a direct update. - Kc Gibson

2 Answers

38
votes

With the following arguments to UpdateItem, you can condition adding a map entry at #number when map.#number does not exist in the map already:

UpdateExpression = "SET map.#number = :string"
ExpressionAttributeNames = { "#number" : "1" }
ExpressionAttributeValues = { ":string" : "the string to store in the map at key value 1" }
ConditionExpression = "attribute_not_exists(map.#number)"
7
votes

Visit http://www.tryyourskill.com/aws/insert-or-append-key-values-map-in-dynamodb-using-java may help you with the code snippet to add or append values in map column

public boolean insertKeyValue(String tableName, String primaryKey, String 
    primaryKeyValue, String updateColumn, String newKey, String newValue) {

    //Configuration to connect to DynamoDB
    Table table = dynamoDB.getTable(tableName);
    boolean insertAppendStatus = false;
    try {
        //Updates when map is already exist in the table
        UpdateItemSpec updateItemSpec = new UpdateItemSpec()
            .withPrimaryKey(primaryKey, primaryKeyValue)
            .withReturnValues(ReturnValue.ALL_NEW)
            .withUpdateExpression("set #columnName." + newKey + " = :columnValue")
            .withNameMap(new NameMap().with("#columnName", updateColumn))
            .withValueMap(new ValueMap().with(":columnValue", newValue))
            .withConditionExpression("attribute_exists("+ updateColumn +")");

        table.updateItem(updateItemSpec);
        insertAppendStatus = true;
    //Add map column when it's not exist in the table
    } catch (ConditionalCheckFailedException e) {
        HashMap<String, String> map =  new HashMap<>();
        map.put(newKey, newValue);
        UpdateItemSpec updateItemSpec = new UpdateItemSpec()
            .withPrimaryKey(primaryKey,primaryKeyValue)
            .withReturnValues(ReturnValue.ALL_NEW)
            .withUpdateExpression("set #columnName = :m")
            .withNameMap(new NameMap().with("#columnName", updateColumn))
            .withValueMap(new ValueMap().withMap(":m", map));

        table.updateItem(updateItemSpec);
        insertAppendStatus = true;
    } catch(Exception e) {
        e.printStackTrace();
    }
    return insertAppendStatus;
}