更新在 dynamodb 中添加属性的项目

Question

Is it not possible to add an attribute in dynamodb dynamically?

I got this error when I tried-" The provided key element does not match the schema ".

Scenario -

{ id : "123",
  imageName : "elephant.jpg"
}

I want to add an attribute - imagePath : "/path/to/image" to the above data. I used put_item, but it replaces the old item if exists.

I am searching for the solution for - If id = "123", then add imagePath attribute else add a new item to the table.

Adding an attribute can be achieved using put_item but it will replace the existing item. How can we dynamically add an attribute to the existing data using update_item ? (appending imagePath to the the given json)

Should I alter the schema of the table with imagePath and then use update_item function?

How can we achieve this using python?

Answer 1

Unfortunately, it can't be achieved in single step. However, it can be achieved in two step process:-

1) Try to insert the data conditionally ie if the key value is already present don't perform any operation (ie insert or update - nothing happens)

2) If there is a ConditionalCheckFailedException , then update the item

Sample code:-

In the below code, usertable is the table name. The key attributes of the table are userid and score . You need to change the below code accordingly for your table structure.

Also, I have assigned the key value (as "Mike"). You need to change it accordingly for your use case.

from __future__ import print_function # Python 2/3 compatibility
from boto.dynamodb2.exceptions import ConditionalCheckFailedException
from botocore.exceptions import ClientError
from boto3.dynamodb.conditions import Attr
import boto3
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='us-west-2', endpoint_url="http://localhost:8000")

table = dynamodb.Table('usertable')

userId = "Mike"

try :
    response = table.put_item(
    Item={
            'userid': userId,
            'score' : 100,
            'imagePath' : '/path/to/image'        
        },
        ConditionExpression=Attr('userid').ne(userId)        
    )

    print("Conditional PutItem succeeded:")
    print(json.dumps(response, indent=4, cls=DecimalEncoder))
except ClientError as ce :    
    print("Conditional check failed:", ce)
    if ce.response['Error']['Code'] == 'ConditionalCheckFailedException':
        print("Key already exists")
        response = table.update_item(
            Key={'userid': userId, 'score' : 100},
            UpdateExpression="set imagePath = :imagePathVal",
            ExpressionAttributeValues={":imagePathVal" : "/path/to/image" }
        )
        print("Update existing item succeeded:")
        print(json.dumps(response, indent=4, cls=DecimalEncoder))        
    else:
        print("Unexpected error: %s" % e

)

Update:-

The data type of variable id and key attribute RequestId should match.

Answer 2

Lastest version of update_item will handle attribute creation if does not already exists

import boto3 
from boto3.dynamodb.conditions import Key

def query_status(asset_id, dynamodb, table):
    try:        
        response = table.query(
            ProjectionExpression="#asset_id, status_id" ,
            ExpressionAttributeNames={"#asset_id": "asset_id"},
            KeyConditionExpression=Key('asset_id').eq(asset_id)        
        )
        if response['Items']:
            return response['Items'][0]["status_id"]
    except:
        pass # if attribute does not exists, return None

    
 
def update_asset_status(asset_id, status_id, dynamodb, table):  
    response = table.update_item(
        Key={'asset_id': asset_id},
        UpdateExpression="set status_id=:r",
        ExpressionAttributeValues={':r': status_id},
        ReturnValues="UPDATED_NEW"
    )
    return response



dynamodb = boto3.resource('dynamodb')
table = dynamodb.Table('assets')

asset_id='1234'

print("Current Status: ", query_status(asset_id, dynamodb, table))

new_status_id='4'
update_asset_status(asset_id, new_status_id, dynamodb, table)

print("New Status: ", query_status(id, dynamodb, table))