ValueObservation by type on generic Record

[Dry-Wet]

Hi.

Let's say I have the following entity:

struct DBEntity<T: Codable & Sendable>: Sendable, Codable, FetchableRecord, PersistableRecord {
    let id: String
    let type: String
    var json: T?

    enum CodingKeys: String, CodingKey {
        case id, type, json
    }
}

The json field can contain any decoded network response.
The type field identifies the response type, for example: String(describing: MyResponse.self).

What I want is to trigger a ValueObservation only when a DBEntity of a certain type changes.
However, what I currently observe is that ValueObservation gets triggered on any write, regardless of the type.

I’ve tried several approaches, such as:

ValueObservation
            .tracking(
                region: DBEntity<T>.filter(Column(DBEntity<T>.CodingKeys.type) == typeid),
                fetch: { db in
                    try DBEntity<T>.fetchOne(db)
                }
            )

And:

ValueObservation
            .tracking { db in
                try DBEntity<T>.filter(Column(DBEntity<T>.CodingKeys.type) == typeid).fetchOne(db)
        }

As well as a couple of other variations and attempts using DatabaseRegionObservation. None of these approaches seem to work as expected.
Am I doing something wrong, or is it simply not possible to achieve this behavior? Or maybe there is a better way to solve this?

Thank you!

[groue]

Hello @Dry-Wet,

We're limited by the abilities of SQLite, which can only track columns and rows identified by their numerical rowid. That's it. Its data change notification callback does not notify the content of the changed rows, preventing GRDB from focusing on changes on a specific entity type.

This is why the ValueObservation documentation mentions:

ValueObservation may notify consecutive identical values.

See #1756 for a recent discussion about the same topic, and possible mitigation techniques.