Using contains filter in DynamoDB scan with Java - android

Question

I have aws dynamo db table as follows.

Now I want to filter it as follows.

For that I used the following code and i get 0 results in my code.what's wrong with my code.how to fix it?

public ScanResult getAllMemos() {
    ScanRequest scanRequest = new ScanRequest()
            .withTableName(DB_NAME)
            .withFilterExpression("contains(imgName,thumbnail)");
    return util.getAmazonDynamoDBClient(getActivity()).scan(scanRequest);
}

Without filter expression I get all results.

The filter expression syntax is quite weird, I think you need to do something like "contains(imgName, :search)" and then additionally call withExpressionAttributeValues("thumbnail") on the ScanRequest, see docs.aws.amazon.com/amazondynamodb/latest/developerguide/… for an example. By the way: you know that scan operations should really not be used in any production environment, it defeats the purpose of dynamodb as a key-value store, it actually is quite "expensive" when your db contains a few million records. — luk2302
– luk2302, Commented Apr 23, 2021 at 18:54
@luk2302 if I use :search this gives error and cannot pass String to withExpressionAttributeValues.it expects a map — Krishan Madushanka
– Krishan Madushanka, Commented Apr 23, 2021 at 18:58
Ah yes, then pass a proper map (as the link) does, have not used the method yet. — luk2302
– luk2302, Commented Apr 23, 2021 at 18:59
expressionAttributeValues.put("imgName", new AttributeValue().withS("thumbnail")); like this? — Krishan Madushanka
– Krishan Madushanka, Commented Apr 23, 2021 at 19:18
Should be ":imgName" if you use "contains(imgName, :imgName)" — luk2302
– luk2302, Commented Apr 23, 2021 at 19:20

JD D · Accepted Answer · 2021-04-24 18:15:59Z

2

Like was discussed in the comments, you need to create an expression attribute value map with the value of the filter and use this.

Try something like this:

public ScanResult getAllMemos() {

    Map<String, AttributeValue> expressionAttributeValues =
        new HashMap<String, AttributeValue>();
    expressionAttributeValues.put(":val", new AttributeValue().withS("thumbnail"));


    ScanRequest scanRequest = new ScanRequest()
            .withTableName(DB_NAME)
            .withFilterExpression("contains(imgName,:val)")
            .withExpressionAttributeValues(expressionAttributeValues);
    return util.getAmazonDynamoDBClient(getActivity()).scan(scanRequest);
}

answered Apr 24, 2021 at 18:15

JD D

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