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From: Matteo Caprari <matteo.caprari@gmail.com>
Date: Wed, 9 Dec 2009 17:55:47 +0000
Message-ID: <1bca98390912090955w65c39cc9vddc1cb1cc750b5cc@mail.gmail.com>
Subject: Help on grouping data by date
To: user@couchdb.apache.org
Content-Type: text/plain; charset=UTF-8

Hello.

I'm learning couchdb and I'd appreciate if someone could have look and
comment on the code and description that follows.

Given a set of documents like {date:"2009-10-11", amount:10500}, I'm
using the view below
and group_level={1,2,3} to group by year, month or day and return the
average amount for each group.

I don't think it's possible to write a rereduce-friendly averaging
function; Instead, I return an object
that holds the total amount and  the number of rows for each group.The
client has to calculate the average on his own.

_view/by_date?group_level=1 returns one row for each year, with the
average amount for that year
_view/by_date?group_level=2 returns one row for each mont, with the
average amount for that month
_view/by_date?group_level=3 returns one row for each day

//map.js:
function(doc) {
	emit(doc.date.split('-'), doc.amount);
}

// reduce.js
function(keys, values, rereduce) {
	if (rereduce) {
		var result = {tot:0, count:0};	
		for (var idx in values) {
			result.tot += values[idx].tot;
			result.count += values[idx].count;
		}
		return result;
	}
	else {
		var result = {tot:sum(values), count:values.length};
		return result;
	}
}

Thanks :)

-- 
:Matteo Caprari
matteo.caprari@gmail.com