Filtering records using keys from an object array - DynamoD

Question

I have an existing data structure in dynamodb where the data is in following format
`myPrimaryKey(ParitionKey) & 
History(attribute)`

```
History: [{
        type: 'type1',
        data: {
            request: {
                addl_vars: {
                    key1: 'value1',
                    key2: 'value2'
                },
                body: {
                    param1: 'value1',
                    param2: 'value2'
                }
            }
        }
    }, {
        type: 'type2',
        data: {
            request: {
                addl_vars: {
                    key1: 'value1',
                    key2: 'value2'
                },
                body: {
                    param1: 'value1',
                    param2: 'value2'
                }
            }
        }
    }
]
```

I need to fetch all objects that satisfies the condition `data.request.addl_vars.key1='value1'`

I have used the below expression to fetch content but ended with errors.

```
{
    TableName: "table",
    FilterExpression: 'contains(**History[*]**, :index)',
    ExpressionAttributeValues: {
        ':index': {
            'data': {
                'request': {
                    'addl_vars': {
                        'key1': 'value1'
                    }
                }
            }
        }
    }
}
```

I tried replacing **History[*]** with **History[0]** and I'm able get only one object. 
The major concern here is that there could be numerous records that satisfy this condition, so can't use keys here.

Can you please help resolving this issue?

Answer

Hi Patton,

To filter items in DynamoDB based on values within a nested list (like the History attribute in your example), you should use instead of *contains* in the *FilterExpression* a combination of projection expressions and filtering on the client side. Here's a high-level approach to achieve your goal:

1. Perform a scan or query operation to retrieve all the items from your DynamoDB table.

2. Use a projection expression to only retrieve the History attribute, as you are interested in filtering within this attribute.

3. In your application code, filter the retrieved items based on the condition you mentioned: data.request.addl_vars.key1 = 'value1'.

Keep in mind that using client-side filtering can be less efficient than performing filtering on the server side using expressions, but in cases where complex nested structures are involved, it may be the most practical approach.

Hope this documentation also helps: 
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Attributes.html#Expressions.Attributes.NestedAttributes

Here you should find more examples on nested attributes: 
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ExpressionAttributeNames.html#Expressions.ExpressionAttributeNames.NestedAttributes

Filtering records using keys from an object array - DynamoD

관련 콘텐츠