Subscription DSL filter causes filtered events to become errors

[justin-m-morgan]

Code of Conduct

• I agree to follow this project's Code of Conduct

AI Policy

• I agree to follow this project's AI Policy, or I agree that AI was not used while creating this issue.

Versions

Elixir - 1.18.4
Erlang - 28.0.2
Ash - 3.5.34
AshGraphql - 1.8.2
Absinthe - 1.7.10

Operating system

MacOS 15.6

Current Behavior

When using the subscription DSL filter, when a filter is applied, the events that out to be excluded are still sent with a nil payload and an error code: not_found.

Example:

    subscription($filter: TestResourceFilterInput) {
      testResourceCreated(filter:$filter) {
        created {
          id
          choice
        }
      }
    }

%Phoenix.Socket.Message{
topic: "__absinthe__:doc:unRcSrcuHkiBCReefuo40W42GErDK62KmQKrtqYnwyU=:1E9C1D467CBCCEB83F768CA9C9AD88068931E40CF6C347A4D1FB07F29F3762C0", 
event: "subscription:data", 
payload: %{result: %{data: %{"testResourceCreated" => nil}, 
errors: [%{code: "not_found", message: "could not be found", path: ["testResourceCreated"], fields: [], vars: [], short_message: "could not be found", locations: [%{line: 2, column: 3}]}]}, 
subscriptionId: "__absinthe__:doc:unRcSrcuHkiBCReefuo40W42GErDK62KmQKrtqYnwyU=:1E9C1D467CBCCEB83F768CA9C9AD88068931E40CF6C347A4D1FB07F29F3762C0"}, ref: nil, join_ref: nil
} 

In the gql playground, this results in just a null event without the included error message which is particularly unhelpful.

{
  "data": {
    "testResourceCreated": null
  }
}

Reproduction

Reproduction Repo

Expected Behavior

When a filter is applied to a subscription, excluded events should not be sent to the subscriber as error events with no data payload.

[justin-m-morgan]

@barnabasJ pointed out in the ash discord that a likely starting point of investigation is the should_send? function in the batcher: https://github.com/ash-project/ash_graphql/blob/d724c46d7d44cd516e26155d144ee7fe3f569867/lib/subscription/batcher.ex

Also, the idea of subscriptions sending null events (in the case of errors) is discussed in the Absinthe repo: absinthe-graphql/absinthe#1309 and absinthe-graphql/absinthe#1181