For a DynamoDB item, how can i append a list which is a value of a key only if the element is not exists in that list?

Question

I have an item in DynamoDB that has a key which has values as a list. I want to append that list with new elements, only if they are not already exists in that list. I don't want to duplicate any element in that list. Item's structure is like below:

{
 "username": "blabla",
 "my_list": ["element1","element2"]
}

I use boto3 library in Python and this is my code block for the update:

response = my_table.update_item(
    Key = {
        'username': "blabla"
    },
    UpdateExpression="SET my_list = list_append(my_list, :i)",
    ExpressionAttributeValues={
        ':i': ["element1"],
    },
    ReturnValues="UPDATED_NEW"
)

I tried to use if_not_exist() in UpdateExpression but always got syntax errors. How can i properly achieve this goal? Thank you.

Answer 1

You should not use a list you need to use a String Set like defined as datatype here:

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html

For adding values to a Set you need to use ADD UpdateOperation like described here

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.ADD

According to this you can change your code like

{
 "username": "blabla",
 "my_list": set(["element1","element2"])
}

# ---

response = my_table.update_item(
    Key = {
        'username': "blabla"
    },
    UpdateExpression="ADD my_list :i",
    ExpressionAttributeValues={
        ':i': set(["element1"]),
    },
    ReturnValues="UPDATED_NEW"
)

This answer shows howto remove items from the set again

https://stackoverflow.com/a/56861830/13055947