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    <p>Your answer is nice, and as I said in the comments, if it works for you, that's all you should care about. Here is an alternative implementation in case you ever bump into performance problems.</p> <p>CouchDB likes tall lists, not fat lists. Instead of view rows keeping an array with every previous tag ever seen, this solution keeps the "sibling" tags in the <strong>key</strong> of the view rows, and then <a href="http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views#Grouping" rel="nofollow">group them together</a> to guarantee one unique sibling tag per row. Every row is just two tags, but there could be thousands or millions of rows: a tall list, which CouchDB prefers.</p> <p>The main idea is to emit a 2-array of tag pairs. Suppose we have one doc, tagged <code>fruit, apple, plant</code>.</p> <pre><code>// Pseudo-code visualization of view rows (before reduce) // Key , Value [apple, fruit ], 1 [apple, plant ], 1 // Basically this is every combination of 2 tags in the set. [fruit, apple ], 1 [fruit, plant ], 1 [plant, apple ], 1 [plant, fruit ], 1 </code></pre> <p>Next I tag something <code>apple, banana</code>.</p> <pre><code>// Pseudo-code visualization of view rows (before reduce) // Key , Value [apple, banana], 1 // This is from my new doc [apple, fruit ], 1 [apple, plant ], 1 // This is also from my new doc [banana, apple], 1 [fruit, apple ], 1 [fruit, plant ], 1 [plant, apple ], 1 [plant, fruit ], 1 </code></pre> <p>Why is the value always <code>1</code>? Because I can make a very simple built-in reduce function: <code>_sum</code> to tell me the count of all tag pairs. Next, query with <code>?group_level=2</code> and CouchDB will give you unique pairs, with a count of their total.</p> <p>A map function to produce this kind of view might look like this:</p> <pre><code>function(doc) { // Emit "sibling" tags, keyed on tag pairs. var tags = doc.tags || [] tags.forEach(function(tag1) { tags.forEach(function(tag2) { if(tag1 != tag2) emit([tag1, tag2], 1) }) }) } </code></pre>
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    1. COBrilliant, thanks Jason. This is much more elegant, and gives me a count too. Thanks for the advice, I'll try to follow the "tall list" principle in future :)
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    2. COI see I can also then query my view for a single key using that `?group=true&startkey=["apple"]&endkey=["apple",{}]` trick.
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    3. COYes, or you could also do `?group_level=1&key=["apple"]` which will bundle up all "apple" tags. CouchDB is relaxed.
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