Create nested maps in DynamoDB

Question

I am trying to create a new map and also assign it a new value at the same time

This is the data format I want to store in my db:

{
    "user_id": 1,
    "project_id": 1,
    "MRR": {
         "NICHE": {
             "define your niche": {
                   "vertical": "test",
                   "ideal prospect": "He is the best"
             }
         },
         "Environment": {
             "Trend 1": {
                   "description": "something"
             },
             "Trend 2": {
                   "description": "something else"
             }
         }

    }

}

My code so far for inserting data is:

def update_dynamo(user_id, project_id, group, sub_type, data):
    dynmoTable.update_item(
        Key = {
            "user_id": user_id
        },
        ConditionExpression=Attr("project_id").eq(project_id),
        UpdateExpression="SET MRR.#group = :group_value",
        ExpressionAttributeNames={
            "#group": group
        },
        ExpressionAttributeValues={
            ":group_value": {}
        }
    )

    dynmoTable.update_item(
        Key={
            "user_id": user_id
        },
        ConditionExpression=Attr("project_id").eq(project_id),
        UpdateExpression="SET MRR.#group.#subgroup = :sub_value",
        ExpressionAttributeNames={
             "#group": group,
             '#subgroup': sub_type
        },
        ExpressionAttributeValues={
             ":sub_value": data
        }
     )
data = {
    "description": "world",
}
if __name__ == "__main__":
    update_dynamo(1, 1, "New Category", "Hello", data)

My question is can these 2 update_items somehow be merged into one?

Nadav Har'El · Accepted Answer · 2020-08-24 16:37:57Z

1

Sure, you can assign to the top-level attribute an entire nested "document", you don't need to assign only scalars.

Something like this should work:

    dynmoTable.update_item(
        Key = {
            "user_id": user_id
        },
        ConditionExpression=Attr("project_id").eq(project_id),
        UpdateExpression="SET MRR.#group = :group_value",
        ExpressionAttributeNames={
            "#group": group
        },
        ExpressionAttributeValues={
            ":group_value": {sub_type: sub_data}
        }
    )

Note how you set the "group" attribute to the Python dictionary {subtype: sub_data}. boto3 will convert this dictionary into the appropriate DynamoDB map attribute, as you expect. You can set sophisticated nested dictionaries, lists, nested in each other this way - in a single update.

answered Aug 24, 2020 at 16:37

Nadav Har'El

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3 Comments

z3y50n Over a year ago

The problem is that when I do your solution it erases the existing data inside an existing map. I would like to also append the data if the "group" already exists.

Nadav Har'El Over a year ago

Oh... In your original question, both the example code you wished to replace, and the text, described replacing whatever MRR.group was by a new map with specific content... Not appending. Appending is a different and more interesting question, and I don't know the answer to it. With a list you can do it, e.g., SET a = list_append(if_not_exists(a, :empty), :val2). Unfortunately, I can't think of a similar function which would work for a map; Also you are not allowed to do two SETS to the same attribute in the same expression.

z3y50n Over a year ago

Yes sorry for misunderstanding. My solution was to create all the groups (Thankfully in my case they are fixed) so I can use the SET expression easily to add maps to each group. Thanks for taking time to answer!

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