DynamoDB ConditionExpression not checking duplicate

Question

I have this DynamoDB table

TableName: MyTable 
    string HashKey: Id,  
    string RangeKey: createTime
Other attributes
    string partId
    string carNum
    string partId_carNum
Local_secondary_index : partId, projection_type       = "ALL"
Local_secondary_index : partId_carNum, projection_type  = "ALL"

I am trying to achieve post with unique combination of Id and partId_carNum. But the conditionExpression is not working. I am seeing duplicate entries in the table. What is wrong in my code?

Here is how my typeScript looks

Below is an example of an item

 {
   "Id" : "000001823841",
   "partId" : "1",
   "carNum" : "car",
   "createTime" : "124232353"
   }



 const params: PutItemInput = {
            TableName: MyTable,
            Item: item,
            ConditionExpression: 'Id <> :f AND partId_carNum <> :g',
            ExpressionAttributeValues: {
                ':f': { 'S': '000001823841' },
                ':g': { 'S': '1#car' }
            }
        };

 return await new DynamoDB()
            .putItem(params)
            .promise()
            .then(() => {
                console.log(`SUCCESS: Event with  ID inserted`);
            })
            .catch(err => {
                console.error(`FAILED to write event to ${params.TableName}. with Error: ${err}`);
            });

Seth Geoghegan · Accepted Answer · 2020-09-28 20:07:06Z

2

DynamoDB does not have a built-in mechanism for ensuring the uniqueness of attributes that are not the primary key.

According to the PutItem docs:

To prevent a new item from replacing an existing item, use a conditional expression that contains the attribute_not_exists function with the name of the attribute being used as the partition key for the table. Since every record must contain that attribute, the attribute_not_exists function will only succeed if no matching item exists

You have a few options here:

Build partId_carNum into your primary key.
Use transactions to simulate the unique constraints in your application code.

Option 2 may feel a bit hackey if you're coming from a RDBMS background, but it's a common pattern in DDB.

answered Sep 28, 2020 at 20:07

Seth Geoghegan

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