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    <p>Since <code>entities.user_mentions</code> is an array, you want to emit a value for each screen_name in the <code>map()</code>:</p> <pre><code>var map = function() { this.entities.user_mentions.forEach(function(mention) { emit(mention.screen_name, { count: 1 }); }) }; </code></pre> <p>Then count the values by unique screen_name in the <code>reduce()</code>:</p> <pre><code>var reduce = function(key, values) { // NB: reduce() uses same format as results emitted by map() var result = { count: 0 }; values.forEach(function(value) { result.count += value.count; }); return result; }; </code></pre> <p>Note: to debug your map/reduce JavaScript functions, you can use <code>print()</code> and <code>printjson()</code> commands. The output will appear in your <code>mongod</code> log.</p> <p>EDIT: For comparison, here is an example using the new <a href="http://docs.mongodb.org/manual/reference/aggregation/" rel="nofollow noreferrer">Aggregation Framework</a> in MongoDB 2.2:</p> <pre><code>db.twitter_sample.aggregate( // Project to limit the document fields included { $project: { _id: 0, "entities.user_mentions" : 1 }}, // Split user_mentions array into a stream of documents { $unwind: "$entities.user_mentions" }, // Group and count the unique mentions by screen_name { $group : { _id: "$entities.user_mentions.screen_name", count: { $sum : 1 } }}, // Optional: sort by count, descending { $sort : { "count" : -1 }} ) </code></pre> <p>The original Map/Reduce approach is best suited for a large data set, as is implied with Twitter data. For a comparison of Map/Reduce vs Aggregation Framework limitations see the related discussion on the StackOverflow question <a href="https://stackoverflow.com/questions/12337319/mongodb-group-group-and-mapreduce/12340283#12340283">MongoDB group(), $group and MapReduce</a>.</p>
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    1. COHats off to you sir. I was wondering can I do the same using the Aggregate Framework? `group()` function?
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    2. CO@chutsu: You can do the same with the Aggregation Framework, but there are a few caveats such as the results being limited to inline at the moment, and to the maximum document size of 16Mb. It may also be possible to use the `group()` command, but this also has some limitations. For a comparison of the approaches and limitations see: [MongoDB group(), $group and MapReduce](http://stackoverflow.com/questions/12337319/mongodb-group-group-and-mapreduce/12340283#12340283).
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    3. CO@chutsu: Added an example with the Aggregation Framework for you ;-)
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