GraphQL tip: Have the subscription types contain all filterable fields on the top-level.

(Read this article on the blog)

For example, a Todo event that should allow filtering by user, group, severity, and id, should have these as well as the item itself:

type TodoEvent {
  userId: ID!
  groupId: ID!
  todoId: ID!
  severity: Severity!
  todo: Todo
}

Originally, it was the only way for filtering in AppSync, but that changed when it started supporting filters on nested fields. So, why it's still a best practice?

The primary reason is deletion: this structure makes it easy to support sending an event when the item is deleted and all the filters will work.

Then an additional benefit is when there is a filter that is not present in the entity itself. For example, if a Todo item's userId can be changed it is useful to notify the original owner. In that case, a prevUserId field can be added to the subscription allowing users to subscribe to events when an item is removed from them.