Filters - AWS AppSync GraphQL
Example

Filters

When querying objects in DynamoDB using the Query and Scan operations, you can optionally specify a filter that evaluates the results and returns only the desired values.

The filter property of a Query or Scan request has the following structure:

type DynamoDBExpression = {
  expression: string;
  expressionNames?: { [key: string]: string};
  expressionValues?: { [key: string]: any};
};

The fields are defined as follows:

expression

The query expression. For more information about how to write filter expressions, see the DynamoDB QueryFilter and DynamoDB ScanFilter documentation. This field must be specified.

expressionNames

The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the expression. The value must be a string that corresponds to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the expression.

expressionValues

The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a “typed value”, see Type system (request mapping). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.

Example

The following example is a filter section for a request, where entries retrieved from DynamoDB are only returned if the title starts with the title argument.

Here we use the util.transform.toDynamoDBFilterExpression to automatically create a filter from an object:

const filter = util.transform.toDynamoDBFilterExpression({
  title: { beginsWith: 'far away' },
});

const request = {};
request.filter = JSON.parse(filter);

This generates the following filter:

{
  "filter": {
    "expression": "(begins_with(#title,:title_beginsWith))",
    "expressionNames": { "#title": "title" },
    "expressionValues": {
      ":title_beginsWith": { "S": "far away" }
    }
  }
}