AWS Boto3 Dynamodb查询问题

Question

I am having issues defining a Query expression for AWS Dynamodb. The goal is to get the unique dynamodb record that has the field timeStamp that contains the string "1475413345". I have confirmed that I can retrieve this via the AWS web console using the same string.

I suspect I have problem with the syntax for .contains incorrect.

Any assistance is greatly appreciated!

Here is the Code:

import boto3
from boto3.dynamodb.conditions import Key, Attr

from time import sleep

benchtemp = boto3.resource('dynamodb')  # if set in ~/.boto
table = benchtemp.Table('benchtemp')

temps = table.query(
    KeyConditionExpression=Key('timeStamp').contains('1475413345')
)


for temp in temps:
    print temp;
    time.sleep(1)

Here is the error message I'm getting:

Traceback (most recent call last):
  File "/Users/neal/PycharmProjects/Read-NMT-DB/Read-NMT-DB.py", line 15, in <module>
    KeyConditionExpression=Key('timeStamp').contains('1475413345')
AttributeError: 'Key' object has no attribute 'contains'

Process finished with exit code 1

Answer 1

KeyConditionExpressions may only constrain the primary (partition and sort) keys of the table/index. If timeStamp is not a primary key, you can use a FilterExpression to filter out the keys that do not match the timestamp prefix you are looking for.

Answer 2

Try writing your query as such:

temps = table.query(
            TableName=table,
            ExpressionAttributeNames='#tS = timeStamp'
            KeyConditionExpression='#tS = :recordTimeStamp',
            ExpressionAttributeValues={":recordTimeStamp": Decimal(1475413345)}
        )

Note that ExpressionAttributeNames is needed because "timeStamp" is a reserved word. For information on that, see here .

Use Decimal if you record timeStamp as a number in your database. To use Decimal, import:

from decimal import *

If you record timeStamp as a string, use str(1475413345) instead of Decimal(1475413345).

As Alexander said, make sure that timeStamp is your partition key. If it is not, either change the table structure so that it is or use a filter expression. You can only query using either partition and sort OR just the partition key.

For example, if userId is your partition key and timeStamp is your sort key, you can perform the following read actions:

Get Item: Find item with userId = ABC and timeStamp = 123
Query by Partition Key: Find all items with userId = ABC
Query by Partition Key and Sort Condition:  Find all items with userId = ABC and timeStamp is < 100

(Source: AWSMobileHub auto-generated examples)