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    copied!<p>It looks a little ugly, but it works:</p> <pre><code>a = [ {"timestamp"=&gt;1347119549, "category"=&gt;nil}, {"timestamp"=&gt;1347119547, "category"=&gt;"Monkeys"}, {"timestamp"=&gt;1347119543, "category"=&gt;nil}, {"timestamp"=&gt;1347119542, "category"=&gt;"Monkeys"}, {"timestamp"=&gt;1347119548, "category"=&gt;"Dog"}, {"timestamp"=&gt;1347119544, "category"=&gt;"Dog"} ] groups = a.sort_by {|h| -h['timestamp']}.group_by {|h| h['category']} sorted = (groups.delete(nil) || []) + groups.values sorted = sorted.sort_by{|i| i.is_a?(Hash) ? -i['timestamp'] : -i.first['timestamp']}.flatten </code></pre> <p>This gives you the following in <code>sorted</code>:</p> <pre><code>[ {"timestamp"=&gt;1347119549, "category"=&gt;nil}, {"timestamp"=&gt;1347119548, "category"=&gt;"Dog"}, {"timestamp"=&gt;1347119544, "category"=&gt;"Dog"}, {"timestamp"=&gt;1347119547, "category"=&gt;"Monkeys"}, {"timestamp"=&gt;1347119542, "category"=&gt;"Monkeys"}, {"timestamp"=&gt;1347119543, "category"=&gt;nil} ] </code></pre> <p>I sort first by <code>'timestamp'</code>, so that the groups are sorted later.</p> <p>After grouping by <code>'category'</code>, I move the values of the <code>nil</code> category in an array. Here, I use <code>(groups.delete(nil) || [])</code> in case the <code>nil</code> group is empty.</p> <p>Now it can be sorted by <code>'timestamp'</code> again, with the <code>timestamp</code> of an array being the <code>timestamp</code> of its first hash.</p> <p>Finally <code>flatten</code> gives us the desired array.</p>
 

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