How do I conditionally insert an item into a dynamodb table using boto3

问题

If I have a table with a hash key of userId and a range key of productId how do I put an item into that table only if it doesn't already exist using boto3's dynamodb bindings?

The normal call to put_item looks like this

table.put_item(Item={'userId': 1, 'productId': 2})

My call with a ConditionExpression looks like this:

table.put_item(
    Item={'userId': 1, 'productId': 2}, 
    ConditionExpression='userId <> :uid AND productId <> :pid', 
    ExpressionAttributeValues={':uid': 1, ':pid': 3}
)

But this raises a ConditionalCheckFailedException every time. Whether an item exists with the same productId or not.

回答1:

The documentation for this unfortunately isn't super clear. I needed to accomplish something similar, and here's what worked for me, using boto3:

try:
    table.put_item(
        Item={
            'foo':1,
            'bar':2,
        },
        ConditionExpression='attribute_not_exists(foo) AND attribute_not_exists(bar)'
    )
except botocore.exceptions.ClientError as e:
    # Ignore the ConditionalCheckFailedException, bubble up
    # other exceptions.
    if e.response['Error']['Code'] != 'ConditionalCheckFailedException':
        raise

Similar to the other answer, the key is in the attribute_not_exists function, but it was unclear to me initially how to get that to work. After some experimentation, I was able to get it going with the above.

回答2:

You dont need the sortkey( or range key) just the partition key or hash key is enough.

try:
    table.put_item(
        Item={
            'foo':1,
            'bar':2,
        },
        ConditionExpression='attribute_not_exists(foo)'
    )
except botocore.exceptions.ClientError as e:
    # Ignore the ConditionalCheckFailedException, bubble up
    # other exceptions.
    if e.response['Error']['Code'] != 'ConditionalCheckFailedException':
        raise

回答3:

i think that you will get better documentation using client.put_item rather than table.put_item

from boto3 documentation:

To prevent a new item from replacing an existing item, use a conditional expression that contains the attribute_not_exists function with the name of the attribute being used as the partition key for the table. Since every record must contain that attribute, the attribute_not_exists function will only succeed if no matching item exists.

ConditionExpression:

ConditionExpression (string) -- A condition that must be satisfied in order for a conditional PutItem operation to succeed.

An expression can contain any of the following:

Functions: attribute_exists | attribute_not_exists | attribute_type | contains | begins_with | size These function names are case-sensitive.

i am using dynamodb2 overwrite parameter on item.save()

来源：https://stackoverflow.com/questions/37053595/how-do-i-conditionally-insert-an-item-into-a-dynamodb-table-using-boto3

标签

python

amazon-dynamodb

boto3