Update DynamoDB item only if the list doesn't contain the value

Question

I’m using AWS SDK to query my DynamoDB table. I would like to update an item based on a conditional expression. If the value is not already existing in a list, I want to add it. 

Here are the params I’ve wrote:

var params = {
  TableName: "table_name",
  Key: {
    PartKey: "part_key",
    SortKey: "sort_key"
  },
  UpdateExpression: "SET MyList = list_append(MyList, :i)",
  ConditionExpression: "NOT contains(MyList, :i)",  
  ExpressionAttributeValues: {
    ":i": [new_value]
  } 
}

Unfortunately, it seems that my conditional expression is always satisfied (true) as even if the value I try to add to my list already exist, the update proceed and I get duplicate...

Answer 1

Your params should look like this:

var params = {
    TableName: "table_name",
    Key: {
      PartKey: "part_key",
      SortKey: "sort_key"
    },
    UpdateExpression: "SET MyList = list_append(MyList, :i)",
    ConditionExpression: "NOT contains(MyList, :b)",  
    ExpressionAttributeValues: {
      ":i": [new_value], 
      ":b": new_value
    } 
}