How to update collections of items in dynamo db?

Question

I have the following table in dynamo:

id | comment_ids | updated_at
1      [12, 123]   '2017-09-09'

when new comment has arrived (like 11 ) how to update the corresponding id to get:

id | comment_ids | updated_at
1   [12, 123, 11]   '2017-09-09'

Answer 1

I'm not a Python developer, but the API you want to use is UpdateItem.

If the data type of the attribute "comment_ids" is a Set:

{
    "TableName": "XXX",
    "Key": {
        "id": {
            "N": "1"
        }
    },
    "UpdateExpression": "ADD #cid :new_cid",
    "ExpressionAttributeNames": {
        "#cid": "comment_ids"
    },
    "ExpressionAttributeValues": {
        ":new_cid": {"N": "11" }
    }
}

If the data type of the attribute "comment_ids" is a List:

{
    "TableName": "XXX",
    "Key": {
        "id": {
            "N": "1"
        }
    },
    "UpdateExpression": "SET #cid = list_append(#cid, :new_cid)",
    "ExpressionAttributeNames": {
        "#cid": "comment_ids"
    },
    "ExpressionAttributeValues": {
        ":new_cid": {"N": "11" }
    }
}

Keep in mind, difference between Set and Lists docs :

Sets Sets can only be of one Scalar type Empty sets are not supported Ordering within the set is not preserved Each value must be unique

Lists A list type attribute can store an ordered collection of values. Lists are enclosed in square brackets: [ ... ]

A list is similar to a JSON array. There are no restrictions on the data types that can be stored in a list element, and the elements in a list element do not have to be of the same type.

Answer 2

Use the operator or function accordingly to append to list or set.

• Operator ADD - if the data type is SET (ie SS)
• Function list_append - if the data type is LIST (ie L)

Appending to List:-

1) Define the list and use it in ExpressionAttributeValues (ie idVals)

idVal = [11]

response = table.update_item(
    Key={
        '#id': 1992            
    },    
    UpdateExpression="SET comment_ids = list_append(comment_ids , :idVal)",
    ExpressionAttributeValues={
        ':idVal': idVal
    },
    ExpressionAttributeNames={
        '#id' : 'id'
    },
)

Appending to Set:-

1) Define the set and use it in ExpressionAttributeValues (ie idVals)

idVals = set([11])

response = table.update_item(
    Key={
        '#id': 1999            
    },
    UpdateExpression="ADD comment_ids :idVal",
    ExpressionAttributeValues={
        ':idVal': idVals
    },
    ExpressionAttributeNames={
        '#id' : 'id'
    },
)