DyanmoDB Scan with Filter

Question

I can't figure out the correct syntax to perform a scan and filter on a table. I want to filter on books written after 2018.

Given this book model:

import { attribute, hashKey, rangeKey, table } from '@aws/dynamodb-data-mapper-annotations';

@table('book')
export class Book {
    @hashKey()
    isbn?: string;

    @rangeKey()
    written?: string; // an ISO-8601 string, e.g. 2020-05-30T16:34:20.985Z

    @attribute()
    title?: string;
}

My scan query filter:

mapper.scan(Book, { filter: '> 2018' }); // what goes here?

Unfortunantly, I get this TypeScript error:

Type 'string' is not assignable to type '(ConditionExpressionSubject & EqualityExpressionPredicate) | (ConditionExpressionSubject & InequalityExpressionPredicate) | ... 15 more ... | undefined'.

What is the correct filter syntax?

mark · Accepted Answer · 2020-05-30 17:31:10Z

0

The AWS DynamoDB lib documentation is terrible but I ended up figuring it out:

import { ConditionExpression } from '@aws/dynamodb-expressions';

// ... later in the file ...

const dateFilter: ConditionExpression = {
    type: 'GreaterThanOrEqualTo',
    subject: 'written',
    object: '2018'
};

const paginator = mapper.scan(Book, { filter: dateFilter });
for await (const book of paginator) {
    console.log(book); // book available here
}

answered May 30, 2020 at 17:31

mark

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Comments

Phil · Accepted Answer · 2021-11-18 07:39:37Z

The filter option must be a ConditionExpression type. According to the official docs, the scan method of mapper takes 1 to 2 arguments. The first argument is the class you want to map to your record in Dynamodb. The second argument takes some options. Filter is one of those options.

The syntax:

    const exp1: ConditionExpression = {
      type: string,
      subject: string,
      object: string
    }

Perform the scan operation:

    mapper.scan(MyClass, {filter: exp1})

If you have more than one condition, you can combine those conditions with compound expressions.

    const exp2: ConditionExpression = {
      type: string,
      subject: string,
      object: string
    }

    const compoundExp: ConditionExpression = {
      type: "And" | "Or" | "Not",
      conditions: [exp1, exp2]
    }

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