DynamoDB attribute_exists doesn't work

Question

Banging my head for a couple hours with this and I can't understand why it's not working 🙁

I have a DynamoDBv2 table (top level attributes are shown as separate columns) with items like these:

{
    gateway: 'gateway1',
    device_id: '2',
    state: { temperature: 20, humidity: 30, pressure: 101049 },
    timestamp: 1498331237261
},
{
    gateway: 'gateway1',
    device_id: '2',
    state: { temperature: 20, humidity: 30, pressure: 101049, lat: 2, long: 1 },
    timestamp: 1498331237262
}

Primary partition key: device_id (String)

Primary sort key: timestamp (Number)

I'm trying to get the latest item for a certain device that has the attributes lat and long.

const docClient = new AWS.DynamoDB.DocumentClient();
const id = 2;
const params = {
    TableName: 'device_telemetry',
    KeyConditionExpression: 'device_id = :id',
    ProjectionExpression: '#s, #lat, #lng, #ts',
    FilterExpression: 'attribute_exists(#lat) AND attribute_exists(#lng)',
    ExpressionAttributeNames: {
        '#s': 'state',
        '#lat': 'state.lat',
        '#lng': 'state.long',
        '#ts': 'timestamp'
    },
    ExpressionAttributeValues: {
        ':id': id.toString()
    },
    ScanIndexForward: false,
        Limit: 1
    };

    const onResult = (res) => {
        console.info('devices res for: ', id, res);
    };

    console.info('query: ', params);
    docClient.query(params).promise().then(onResult);

If I use attribute_exists(#s), then it works, but for #lat or #long it doesn't 🙁

What am I doing wrong? 😖

Answer 1

You can not directly access nested attributes. You should declare in this format

 ExpressionAttributeNames: {
    '#s': 'state',
    '#lat': 'lat',
    '#lng': 'long',
    '#ts': 'timestamp'
},

and access the attributes in following way:

FilterExpression: 'attribute_exists(#s.#lat) AND attribute_exists(#s.#lng)',

Answer 2

Seems I was mixing item attributes with the attributes of the map value of state. It was confusing when I was looking at the generated result.

Sorry for the noise.