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Date: Thu, 5 Feb 2009 16:23:15 +0000
Message-ID: <214c385b0902050823vc9c7f3fld0f0d4245ad296ca@mail.gmail.com>
Subject: Re: Reduce to nothing
From: Matt Goodall <matt.goodall@gmail.com>
To: user@couchdb.apache.org
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2009/2/5 Michael Marks <michael.marks@gmx.de>:
> How about having a third attribute "state": with possible values "active",

Sure, that's possible but it defeats the point of my original post,
i.e. not touching the original document.

Just to be clear, I know you can stick any old attribute on a "thing"
document and filter it in a view. What I'm interested in is using the
reduce function in more interesting ways that /may/ have additional
benefits in some cases.

- Matt

> Matt Goodall schrieb:
>>
>> 2009/2/5 Jeremy Wall <jwall@google.com>:
>>
>>>
>>> Can't you just have the map screen out the cancellations? Seems like
>>> that's
>>> the most natural spot for the behaviour you want.
>>>
>>
>> Nope, because the "thing" doesn't know it's been cancelled.
>>
>> - Matt
>>
>>
>>
>>>
>>> On Feb 5, 2009 8:34 AM, "Matt Goodall" <matt.goodall@gmail.com> 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
>>>
>>>
>>
>>
>