How can I filter a subscription using a custom resolver - aws-amplify

Question

I am working on a messaging app using AWS AppSync.
I have the following message type...
type Message
#model
#auth(
rules: [
{ allow: groups, groups: ["externalUser"], operations: [] }
]
) {
id: ID!
channelId: ID!
senderId: ID!
channel: Channel #connection(fields: ["channelId"])
createdAt: AWSDateTime!
text: String
}
And I have a subscription onCreatemessage. I need to filter the results to only channels that the user is in. So I get a list of channels from a permissions table and add the following to my response mapping template.
$extensions.setSubscriptionFilter({
"filterGroup": [
{
"filters" : [
{
"fieldName" : "channelId",
"operator" : "in",
"value" : $context.result.channelIds
}
]
}
]
})
$util.toJson($messageResult)
And it works great. But if a user is in more than 5 channels, I get the following error.
{
"message": "Connection failed: {"errors":[{"message":"subscription exceeds maximum value limit 5 for operator `in`.","errorCode":400}]}"
}
I am new to vtl. So my question is, how can I break up that filter in to multiple or'd filters?

Answer 1

According to Creating enhanced subscription filters, "multiple rules in a filter are evaluated using AND logic, while multiple filters in a filter group are evaluated using OR logic".
Therefore, as I understand it, you just need to split $context.result.channelIds into groups of 5 and add an object to the filters array for each group.
Here is a VTL template that will do this for you:
#set($filters = [])
#foreach($channelId in $context.result.channelIds)
#set($group = $foreach.index / 5)
#if($filters.size() < $group + 1)
$util.qr($filters.add({
"fieldName" : "channelId",
"operator" : "in",
"value" : []
}
))
#end
$util.qr($filters.get($group).value.add($channelId))
#end
$extensions.setSubscriptionFilter({
"filterGroup": [
{
"filters" : $filters
}
]
})
You can see this template running here: https://mappingtool.dev/app/appsync/042769cd78b0e928db31212f5ee6aa17
(Note: The Mapping Tool errors on line 15 are a result of the $filters array being dynamically populated. You can safely ignore them.)

Answer 2

Do you want to add server-side filter for GraphQL Subscriptions?
If so, Now, Amplify is supported for server-side filter for Subscriptions.
After you checking below blog, you may sense it.
https://aws.amazon.com/blogs/mobile/announcing-server-side-filters-for-real-time-graphql-subscriptions-with-aws-amplify/