Nested attributes do not work when filtering events

[michelalbers]

Assume a filter:

filter: (_, _, context) => ({
    update: {
       userId: "foo"
    }
})

Which will be converted to the dynamodb query:

{
      TableName: "graphql_subscriptions",
      IndexName: "TopicIndex",
      ExpressionAttributeNames: {
        "#hashKey": "topic",
        "#filter": "filter",
        "#0": "update.userId",
      },
      ExpressionAttributeValues: {
        ":hashKey": "onUserBadgeCountChange",
        ":0": "foo",
      },
      KeyConditionExpression: "#hashKey = :hashKey",
      FilterExpression:  "(#filter.#0 = :0  OR attribute_not_exists(#filter.#0))",
}

Which is incorrect. As a result the filter will not work and all subscribers will receive the message.

Nested filters have to be written like this:

{
      TableName: "graphql_subscriptions",
      IndexName: "TopicIndex",
      ExpressionAttributeNames: {
        "#hashKey": "topic",
        "#filter": "filter",
        "#0": "update",
        "#1": "userId" // split the key by "."
      },
      ExpressionAttributeValues: {
        ":hashKey": "onUserBadgeCountChange",
        ":0": "foo",
      },
      KeyConditionExpression: "#hashKey = :hashKey",
      FilterExpression:  "(#filter.#0.#1 = :0  OR attribute_not_exists(#filter.#0.#1))", // Join the key back together with #0.#1
}

A very early draft of a solution inside getFilteredSubs.ts looks like this:

for (const [key, value] of Object.entries(flattenPayload)) {
  const aliasNumber = attributeCounter++;
  let subAliasNumbers = [];
  let subAliasNumber = aliasNumber;
  for (const part of key.split('.')) {
    expressionAttributeNames[`#${subAliasNumber}`] = part;
    subAliasNumbers.push(subAliasNumber);
    subAliasNumber++;
  }
  expressionAttributeValues[`:${aliasNumber}`] = value;
  filterExpressions.push(`(#filter.${subAliasNumbers.map(n => `#${n}`).join('.')} = :${aliasNumber} OR attribute_not_exists(#filter.${subAliasNumbers.map(n => `#${n}`).join('.')})`);
}

Unfortunetaly the docs are misleading as well - nested values are clearly described as "working fine" when not working at all.