how to get certain columns from dynamodb aws - Amazon Web Sevices

matchaLover
April 1, 2023
175 views
2 votes
3 Answers

I want to get only timestamp, address, distance, and proximity from DynamoDB using Lambda function, so that I can get the response at API Gateway to display on my React Native app.
May I know what’s wrong with my Lambda function code?

import boto3
import json

from boto3.dynamodb.conditions import Key, Attr

def lambda_handler(event, context):

    IRAPID_BEACON = 'irapid_beacon'
    
    DDB_RESOURCE = boto3.resource('dynamodb')
    IRAPID_BEACON_DDB_TABLE = DDB_RESOURCE.Table(IRAPID_BEACON)
    
    result = IRAPID_BEACON_DDB_TABLE.get_item(Key={'timestamp'}, ProjectionExpression='#timestamp, address, distance, proximity', ExpressionAttributeNames={'#address': 'address'})
    
    return {
        'body': result
    }

This is the response I get:

{
  "errorMessage": "Parameter validation failed:nInvalid type for parameter Key, value: {'timestamp'}, type: <class 'set'>, valid types: <class 'dict'>",
  "errorType": "ParamValidationError",
  "requestId": "3af15dff-d323-49bf-9429-bfce2d26e579",
  "stackTrace": [
    "  File "/var/task/lambda_function.py", line 14, in lambda_handlern    result = IRAPID_BEACON_DDB_TABLE.get_item(Key={'timestamp'}, ProjectionExpression='#timestamp, address, distance, proximity', ExpressionAttributeNames={'#address': 'address'})n",
    "  File "/var/runtime/boto3/resources/factory.py", line 520, in do_actionn    response = action(self, *args, **kwargs)n",
    "  File "/var/runtime/boto3/resources/action.py", line 83, in __call__n    response = getattr(parent.meta.client, operation_name)(*args, **params)n",
    "  File "/var/runtime/botocore/client.py", line 391, in _api_calln    return self._make_api_call(operation_name, kwargs)n",
    "  File "/var/runtime/botocore/client.py", line 691, in _make_api_calln    request_dict = self._convert_to_request_dict(n",
    "  File "/var/runtime/botocore/client.py", line 739, in _convert_to_request_dictn    request_dict = self._serializer.serialize_to_request(n",
    "  File "/var/runtime/botocore/validate.py", line 360, in serialize_to_requestn    raise ParamValidationError(report=report.generate_report())n"
  ]
}

Answers

Chosen as BEST ANSWER

I solved my problem by this:

import boto3
import json

from boto3.dynamodb.conditions import Key, Attr
    
def lambda_handler(event, context):

    dynamodb = boto3.client('dynamodb')
    
    IRAPID_BEACON = 'irapid_beacon'
    DDB_RESOURCE = boto3.resource('dynamodb')
    IRAPID_BEACON_TABLE = DDB_RESOURCE.Table(IRAPID_BEACON)
    
    projection_expression = '#attr1, #attr2, #attr3, #attr4'
    
    expression_attribute_names = {
        '#attr1': 'timestamp',
        '#attr2': 'address',
        '#attr3': 'distance',
        '#attr4': 'proximity',
    }
    
    scan_params = {
        'TableName': IRAPID_BEACON,
        'ProjectionExpression': projection_expression,
        'ExpressionAttributeNames': expression_attribute_names
    }

    response = dynamodb.scan(**scan_params)
    
    data = response['Items']
    while 'LastEvaluatedKey' in response:
        response = IRAPID_BEACON_TABLE.scan(ExclusiveStartKey=response['LastEvaluatedKey'])
        data.extend(response['Items'])
        
    return data

The result I got:

[
  {
    "proximity": {
      "S": "IMMEDIATE"
    },
    "address": {
      "S": "E3:60:15:DA:38:E2"
    },
    "distance": {
      "N": "0.013031512640464739"
    },
    "timestamp": {
      "N": "1679536528228"
    }
  },
  {
    "proximity": {
      "S": "FAR"
    },
    "address": {
      "S": "C7:EE:CA:83:FA:AC"
    },
    "distance": {
      "N": "60.536073313879164"
    },
    "timestamp": {
      "N": "1679476131297"
    }
  }, ...]

(Edit)

- lpizzinidev
- April 1, 2023 at 9:00 am
- 0 votes
0
As suggested by the error message, you should specify a value in your Key parameter:
```
Key={'timestamp': '1680332401551'}
```
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- LeeHannigan
- April 1, 2023 at 12:29 pm
- 0 votes
0
I suggest that you take a deep dive into DynamoDB documentation. I don’t believe your data model will do what you expect.

If you want to do a GetItem you must provide the value of the partition key, in your case the timestamp.
```
Key={'timestamp': '<some time stamp value>'}
```
Its important to note that having timestamp as the partition key provides you very little query flexibility, it is also an issue if 2 items are written at the same time, one item would overwrite the other.

With your current schema, you would need to do a Scan with a FilterExpression which would be highly inefficient.

My suggestion depending on your throughput would be to use a static partition key value and timestamp as sort key. This will allow you do things like "give me all the items between this time range" or "give me the item at this specific time"

PK SK data

1 1680332401000 other

1 1680332412345 other

1 1680332402222 other

Using Query you can now do things like:
```
SELECT * FROM mytable WHERE PK=1 AND SK > 1680332401000
```
Note that this would assume you do not write to the table more than 1000 RPS. If you do, then you would need to shard the PK by #no of reqs/1000.
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