How to get name of original stream from projection's event object?

EventStoreDB
1

Anywhere I can read about the various properties and structures in the “e” object (i.e. the event) for a projection.

I’m trying to do something like the below sample: create a “header” stream for each Widget stream. Widget streams are full of tons of low-level events, but when I want to just build up quick header information, I only care about a small subset of those events, so I want to create a companion “header” stream.

But how do I get the name of the original stream ( so I can extract the Widget ID from it? )

fromCategory(“Widget”)

.foreachStream()

.when({

WidgetCreated: function(s,e) {
var origStream = e.??? // HOW DO I GET THIS?
var widgetId = origStream.substring(7); // i.e. Widget-59e9a5f8-ac1b-41d9-8230-728a6be937e9
linkTo(“Table.header-” + widgetId, e);
},

WidgetAltered: function(s,e) { … /* ditto */ … },

WidgetDeleted: function(s,e) { … /* ditto */ … }

});

``

2

Just as a helper, you know you can debug projections in chrome and see
all this information in the debugger? Old UI but works very similarly
now https://www.youtube.com/watch?v=NFnsFfOMMkI

I'll let pieter follow up I am not sure if the stream id is exposed
there, that said in this case shouldn't Id be part of the event
itself?

3

Based on the screenshot from that YouTube it appears “streamId” is the property I was looking for.

But no, I hadn’t tried debugging in Chrome. Good to know…

4

It should still be in the event in this case. WidgetCreated but no Id
in the event? You can hack through it in a projection but what about
another consumer? Also if you use a linkTo on it it can appear in a
different stream (say foobarbaz) where you wouldn't be able to parse
the stream name to get the id.

5

The streams in the “Widget” category should all be originating streams, not projection streams with linked events.

Sure, in WidgetCreated, I can grab the id from there, but the other events I can only extract the widget UUID from the stream name, so hence “streamId”.

6

"Sure, in WidgetCreated, I can grab the id from there, but the other
events I can only extract the widget UUID from the stream name, so
hence "streamId". "

Normally you would want widget id in the event not just the streamid
if for no other reason than you dont want every consumer to have to
parse a stream name

7

Fair enough. Thanks.

8

Hi Zach, apologies for only getting back to you now.
If you want to know what properties are on the event for the handler, you can find them here. https://github.com/EventStore/EventStore/blob/release-v4.0.0/src/EventStore.Projections.Core/Prelude/Projections.js#L267