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POMongo aggregation framework, Sort and then group not working
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  1. POMongo aggregation framework, Sort and then group not working
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    copied!<p>I am trying sort data by date first and then group on another field. It is not working to me.</p> <p><em><strong>The question I am trying to answer is: Select the most recent distinct cid?</em></strong></p> <p>given this data:</p> <pre><code>db.summary.save({"lid" : 5, "date" : 5, "cid" : 2, "circles" : [ 2 ] }) db.summary.save({"lid" : 2, "date" : 2, "cid" : 1, "circles" : [ 2 ] }) db.summary.save({"lid" : 4, "date" : 0, "cid" : 3, "circles" : [ 2 ] }) db.summary.save({"lid" : 3, "date" : 3, "cid" : 2, "circles" : [ 2 ] }) db.summary.save({"lid" : 1, "date" : 1, "cid" : 1, "circles" : [ 2 ] }) db.summary.aggregate( {$match :{circles: 2}, $sort: {date: -1}, $group: {_id: '$cid'}} ) </code></pre> <p>I am doing a match first on circles, then a sort on date, then a group on cid</p> <p>The result I am getting: </p> <pre><code>{ "result" : [ { "_id" : 3 }, { "_id" : 1 }, { "_id" : 2 } ], "ok" : 1 } </code></pre> <p><strong>Here is my analysis:</strong></p> <p>before matching or sorting by date, the data was:</p> <pre><code>"lid" : 5, "date" : 5, "cid" : 2 "lid" : 2, "date" : 2, "cid" : 1 "lid" : 4, "date" : 0, "cid" : 3 "lid" : 3, "date" : 3, "cid" : 2 "lid" : 1, "date" : 1, "cid" : 1 </code></pre> <p>After sorting by date, the data set would be:</p> <pre><code>"lid" : 5, "date" : 5, "cid" : 2 "lid" : 3, "date" : 3, "cid" : 2 "lid" : 2, "date" : 2, "cid" : 1 "lid" : 1, "date" : 1, "cid" : 1 "lid" : 4, "date" : 0, "cid" : 3 </code></pre> <p>So after grouping, the result I expect is:</p> <pre><code>{ "result" : [ { "_id" : 2 }, { "_id" : 1 }, { "_id" : 3 } ], "ok" : 1 } </code></pre> <p>What query does solve my problem?</p> <p>Why is the current query not working for me?</p>
 

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