Adding generic for payload type to PgNotification

[LucaCappelletti94]

At this time, PgNotification does not have a serde Deserializable generic which defines the type of the payload, but it is left to a String. Would it be reasonable to add it?

diesel/diesel/src/pg/backend.rs

Lines 164 to 176 in 94d5a0d

	pub struct PgNotification {
	/// process ID of notifying server process
	pub process_id: i32,
	/// Name of the notification channel
	pub channel: String,
	/// optional data that was submitted with the notification,
	///
	/// This is set to an empty string if no data was submitted
	///
	/// (Postgres unfortunally does not provide a way to differentiate between
	/// not set and empty here)
	pub payload: String,
	}

[weiznich]

This is expect, because the payload is really just a string in the general case. It might be the case that the string often contains a json payload, but that's something applications build on top of this feature and therefore something that should be implemented in these applications itself.

[LucaCappelletti94]

While I do agree that the default should be String, maybe using something like the following would avoid the need to creating a wrapper struct. What do you think?

 pub struct PgNotification<Payload = String> { 
     /// process ID of notifying server process 
     pub process_id: i32, 
     /// Name of the notification channel 
     pub channel: String, 
     /// optional data that was submitted with the notification, 
     /// 
     /// This is set to an empty string if no data was submitted 
     /// 
     /// (Postgres unfortunally does not provide a way to differentiate between 
     /// not set and empty here) 
     pub payload: Payload, 
 } 

[weiznich]

It's hard how this exactly would work, especially given that we somehow would need to initialize the custom payload field. Also it's unclear for me in how far that would minimize the need for creating a wrapper struct as you still would need to define a custom type for Payload. Or to word it differently: Could you explain in detail why this should be simpler/better to use than just calling let your_payload: Payload = serde::from_str(&notification.payload)?; in your application code?

[LucaCappelletti94]

You are right, it does not really clean up thing in a semantically clear way.