dynamoDB conditional write only when there are N or less items in a map

Question

We are storing a map in DynamoDB (not my design choice). So, basically a key can contain a list of values. Something like map[string][]someStruct.

We can only append a value to a given key if there are only N or less values for a given key. For example, if "key1" already has 3 values, I cannot append another value, but if it has less than 3 values, I can append one more value.

I looked at Conditional Writes, but couldn't find a conditional expression that would help with this. Any help is greatly appreciated. Thanks!

Answer 1

You would need to store the number of records in the list along side the list, perhaps something like this would work:

  {
    "key1": {
      "list": [1,2,3,4],
      "listLength": 4
    }
  }

You'll need to make sure that the list and listLength are kept in sync.

Alternatively you can get the item, check the length and then update with a condition to make sure it hasn't been updated between you get and update operation.