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From Paul Davis <>
Subject Re: Reduce to nothing
Date Thu, 05 Feb 2009 17:06:46 GMT

About the best you're going to get with the current implementation
would be similar to the code that Jeremy posted which in effect
returns a 'cancelled' value for that item. I did just wake up so I
might be missing something clever, but I'm pretty sure this is baked
in because of the incremental calculation stuff.

Paul Davis

On Thu, Feb 5, 2009 at 9:33 AM, Matt Goodall <> wrote:
> Hi,
> Is it possible to reduce a key to nothing, i.e. completely remove a
> key from the reduction result.
> For instance, say you post three documents:
>    {"_id": "thing1", "type": "thing"}
>    {"_id": "thing2", "type": "thing"}
>    {"_id": "...", "type": "cancellation", "cancels": "thing1"}
> It's trivial to produce a map function that collates the "thing" and
> "cancellation" documents. However, I can't work out how, or even it
> it's possible, to reduce the view that so that only "thing2" remains.
> This seems like it might be a useful thing to be able to do at times.
> For instance, posting a "cancellation" document effectively removes a
> "thing" from the database. Nothing needs to touch the "thing",
> avoiding any chance of conflicts and it doesn't matter how many
> "cancellation" documents get posted making cancellation an idempotent
> operation.
> I tried not returning anything, just in case it worked ;-), but got a
> JSON encoding error (can't encode undefined, iirc). That would be a
> simple approach. However, I wondered if the more "normal" approach of
> allowing a reduce function to emit zero or more (key, value) pairs
> would be even better? Not sure if that would break the incremental
> reduce though.
> Any ideas, or is there another way to achieve this that I'm missing?
> - Matt

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