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From J Chris Anderson <jch...@apache.org>
Subject Re: Testing map/reduce functions
Date Mon, 13 Sep 2010 18:26:57 GMT

On Sep 13, 2010, at 9:11 AM, Thomas Rampelberg wrote:

> It would be epically awsome to have unit test functionality for
> map/reduce baked into couchapp .....
> 

On my infinite todo list is the idea to implement CouchApp in Node.js, which would make adding
extensions like this "trivial."

I doubt that will ever get to the top of my list, so if someone else takes it on that'd be
rad. Work should probably be coordinated on the CouchApp list:

http://groups.google.com/group/couchapp/

Chris

> On Mon, Sep 13, 2010 at 8:49 AM, Mark J. Reed <markjreed@gmail.com> wrote:
>> On Mon, Sep 13, 2010 at 11:08 AM, PaweĊ‚ Stawicki
>> <pawelstawicki@gmail.com> wrote:
>>> You can take a look on my blog post:
>>> http://pawelstawicki.blogspot.com/2009/12/couchdb.html
>>> At the end are reduce parameters explained.
>> 
>> Nitpick: this reduce call given in your post is invalid:
>> 
>>> function(["developer", "manager", "administrator"], [17, 4, 2], true);
>> 
>> Because when rereduce is true, the first parameter (keys) is presumably null.
>> 
>> Basically, a map function emits a (key, value) pair; the plumbing
>> automatically adds the document id that was being processed when the
>> pair was emitted, so what you get out of the map phase is essentially
>> a bunch of triples: (key1, value1, document1), (key2, value2,
>> document2), etc.
>> 
>> The reduce function, pre-rereduce, gets some subset of those triples,
>> slightly re-organized.  A single call to reduce might look like this:
>> 
>> reduce([[key1, document1], [key2, document2], [value1, value2], false)
>> 
>> Unlike classical map/reduce, the reduce function is not guaranteed to
>> get all of the values associated with a given key.  Nonetheless, it is
>> expected to return a single result value (which may be a complex value
>> like an array or object).
>> 
>> The results of the reduce function may be passed back into another
>> reduce pass.  In this case, there are no keys, values, or documents,
>> just a single array of results from a previous call to the same reduce
>> function.
>> 
>> reduce(null, [reduced1, reduced2, reduced3], true)
>> 
>> -
>> Mark J. Reed <markjreed@gmail.com>
>> 


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