Projections - how to combine two (or more) categories into a single stream?

EventStoreDB
#1

So I have effectively created a single stream for a category using a projection.

Given the streams:

Foobar-1

Foobar-2

Foobar-3

Foobar-4

I can create a single stream containng all events from those streams by doing the following:

fromCategory(‘Foobar’)

.whenAny(function(s,e) {

linkTo('Foobars',e);

});

This gives me a “Foobars” stream with all events from the four streams listed above

If I want to do this across multiple categories, I am stuck. Let’s say I have the following streams:

Foobar-1

Foobar-2

Barfoo-1

Barfoo-2

How could I create a projection to capture all events across both of those two categories (but no other categories)? I was looking for a “fromCategories” function (like “fromStreams”) but couldn’t find one.

Cheers,

Callum

#2

fromAll and parse the stream name?

#3

This construction works for multiple streams (we use it already):

fromStreams([‘Foobar-1’, ‘Foobar-2’, ‘Barfoo-1’, ‘Barfoo-2’])

.whenAny(function(s,e) {

linkTo('resultStream',e);

});

Or this is not what you want?

#4

I wanted to do it for categories, so "fromStreams" is not so useful. I
don't know all the streams upfront.

While I am sure Greg's solution works, I realised that if I layer some
projections correctly I can do it that way too.

So if I have streams...

Foobar-1
Foobar-2
Barfoo-1
Barfoo-2

Then...

Projection 1 reads the "Foobar" category and writes stream -
"AllTheThings-Foobar".
Projection 2 reads the "Barfoo" category and writes stream -
"AllTheThings-Barfoo".
Projection 3 reads the "AllTheThings" caegory and writes stream
"AllTheThings".

That's pretty useful in my case. The "interim" streams produced by
Projections 1 and 2 give me some more information.

Callum

#5

I realise this post is old, but I needed to do the same thing recently. I solved it by using fromStreams with the system defined category streams, e.g.

fromStreams(["$ce-category1", “$ce-category2”])

.when({

$any: function(s, e) {

linkTo(“MyStream”, e)

}

});

#6

for a small number of streams this works great, as the number rises
fromAll is better