如何使用boto3有条件地将项目插入到dynamodb表中

问题描述

如果我有一个表，其中的哈希键为userId，范围键为productId，那么仅当使用boto3的dynamodb绑定不存在该项目时，才将其放入该表中吗？

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?

对put_item的正常调用如下所示：

The normal call to put_item looks like this

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

使用ConditionExpression进行的呼叫如下：

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}
)

但这每次都会引发ConditionalCheckFailedException。是否存在具有相同productId的商品。

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

推荐答案

不幸的是，有关此文件的文档并不十分清楚。我需要使用boto3完成类似的操作，这对我有用：

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

类似于其他答案，关键是在attribute_not_exists函数中，但是最初我不清楚如何使它起作用。经过一番试验，我得以将其与上述方法结合使用。

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.

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