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POUsing a CouchDB view, can I count groups and filter by key range at the same time?
 

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  1. POUsing a CouchDB view, can I count groups and filter by key range at the same time?
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    <p>I'm using CouchDB. I'd like to be able to count occurrences of values of specific fields within a date range that can be specified at query time. I seem to be able to do parts of this, but I'm having trouble understanding the best way to pull it all together.</p> <p>Assuming documents that have a timestamp field and another field, e.g.:</p> <pre><code>{ date: '20120101-1853', author: 'bart' } { date: '20120102-1850', author: 'homer'} { date: '20120103-2359', author: 'homer'} { date: '20120104-1200', author: 'lisa'} { date: '20120815-1250', author: 'lisa'} </code></pre> <p>I can easily create a view that <strong>filters documents by a flexible date range</strong>. This can be done with a view like the one below, called with key range parameters, e.g. <code>_view/all-docs?startkey=20120101-0000&amp;endkey=20120201-0000</code>.</p> <p><em>all-docs/map.js:</em></p> <pre><code>function(doc) { emit(doc.date, doc); } </code></pre> <p>With the data above, this would return a CouchDB view containing just the first 4 docs (the only docs in the date range).</p> <p>I can also create a query that <strong>counts occurrences of a given field</strong>, like this, called with grouping, i.e. <code>_view/author-count?group=true</code>:</p> <p><em>author-count/map.js:</em></p> <pre><code>function(doc) { emit(doc.author, 1); } </code></pre> <p><em>author-count/reduce.js:</em></p> <pre><code>function(keys, values, rereduce) { return sum(values); } </code></pre> <p>This would yield something like:</p> <pre><code>{ "rows": [ {"key":"bart","value":1}, {"key":"homer","value":2} {"key":"lisa","value":2} ] } </code></pre> <p>However, I can't find the best way to <strong>both filter by date and count occurrences</strong>. For example, with the data above, I'd like to be able to specify range parameters like <code>startkey=20120101-0000&amp;endkey=20120201-0000</code> and get a result like this, where the last doc is excluded from the count because it is outside the specified date range:</p> <pre><code>{ "rows": [ {"key":"bart","value":1}, {"key":"homer","value":2} {"key":"lisa","value":1} ] } </code></pre> <p>What's the most elegant way to do this? Is this achievable with a single query? Should I be using another CouchDB construct, or is a view sufficient for this?</p>
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    1. COMy first thought would be to query the view for your date range, then use a [_list function](http://wiki.apache.org/couchdb/Formatting_with_Show_and_List#Listing_Views_with_CouchDB_0.10_and_later) to do the group/counts. You can't operate on multiple keys within the same query like that, so you'll need that 2nd layer.
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      1. USDominic Barnes
 

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