Dynamodb DB scan: Filter on embedded object in array

Question

Trying to be able to filter on embedded object that looks like:

    "posts": [
        {
            "id": "10e85cf7-acd2-417b-a5dc-1dfb6de606bf",
            "references": [
                {
                    "type": "URL",
                    "title": "How to get dynamodb to only return certain columns",
                },
                {
                    "type": "HASHTAG",
                    "title": "#dynamodb",
                },
            ]
        },
...
]

I am trying to return all posts that have reference of type "HASHTAG" and VALUE "#dynamodb". I have tried this but it always returns null (running in node.js):

const params = {
  TableName: "tableName",
  ScanIndexForward: false,
  FilterExpression: "#references.#type = :referenceValue",
  ExpressionAttributeNames: {
    "#references": "references",
    "#type": "HASHTAG"
  },
  ExpressionAttributeValues: {
      ":referenceValue": "#dynamoDB"
  }
}
const response = await docClient.scan(params).promise();
console.log(response);

And only returns (successfully) an empty array.

fedonev · Accepted Answer · 2022-05-28 09:30:15Z

2

Use the contains function to filter on items in a list. In your case, return posts that have an item like :map at the path indicated by #references.

FilterExpression: 'contains(#references, :map)',
ExpressionAttributeNames: { '#references': 'references' },
ExpressionAttributeValues: {
  ':map': {
    type: 'HASHTAG',
    title: '#dynamodb',
  },
},

answered May 28, 2022 at 9:30

fedonev

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3 Comments

Andyrewwer Over a year ago

Actually a follow-up question: imagining I had a more complex object with more fields than just "title" (e.g. "text"), is it possible to check if the type & title match and ignore all other fields? Looking at documentation it seemed maybe not.

fedonev Over a year ago

@Andyrewwer You are right, no partial or wildcard matching. The map either matches or doesn't. The best you can do to combine multiple contains functions with an OR operator.

fedonev Over a year ago

@Andyrewwer Also, remember a scan with a filter is still a scan. The filter is applied after reading all records. This means that having to further filter the results client-side is probably not adding much DynamoDB expense.

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