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copied!<p>In CouchDB you would write a map/reduce that outputs ALL of the cars and drivers with complex keys, and then use key ranges to pick both. For example, let's assume your documents look like these two...</p> <pre><code>{ "_id": "...", "_rev": "...", "docType": "driver" } { "_id": "...", "_rev": "...", "docType": "car", "driver": "driver's _id" } </code></pre> <p>You could use <a href="http://en.wikipedia.org/wiki/Duck_typing" rel="nofollow noreferrer">duck typing</a> instead of specifying the docType, but I like this method better.</p> <p>Your map function:</p> <pre><code>function(doc) { if(doc.docType == "driver") emit([doc.id, 0], doc); elseif(doc.docType == "car") emit([doc.driver, 1], doc]; } </code></pre> <p>Our complex key is an array, with the first item always being the driver's _id. The second item in the array prevents key collision, and allows us to reference the car or driver directly (more on this later). </p> <p>We can now use the key range query parameters to grab both of the docs.</p> <pre><code>?startkey=["driver _id"]&endkey=["driver _id", {}] </code></pre> <p>This basically says "give me any array with the driver _id as the first item, and anything in the second. This works because objects - the second item in the <code>endkey</code>'s array - is sorted as the highest. See <a href="http://wiki.apache.org/couchdb/View_collation?redirect=ViewCollation#Collation_Specification" rel="nofollow noreferrer">http://wiki.apache.org/couchdb/View_collation?redirect=ViewCollation#Collation_Specification</a> for more information about how items get sorted/weighed in keys.</p> <p>This also scales quite nicely, because we can add more information into our map function without having to change our query in the client. Let's say we add a sponsor docType: we just add another <code>elseif</code> for the docType field and then <code>emit([doc.driver, 2], doc);</code>. Now we can pull all three documents in one request with the same key range query from above.</p> <p>Of course, you can also specify individual documents instead of pulling all of them. <code>?key=["driver's _id", 1]</code> would pull just the car for the specified driver.</p> <p>Cheers.</p>

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