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POMongo DB - Sort and Skip within a group
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  1. POMongo DB - Sort and Skip within a group
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    copied!<p>I am using MongoDB and I want to sort and skip records within a group.</p> <p>Below is a sample data:</p> <pre><code>{ "_id" : ObjectId("51cd7274267d959cb9f95cea"), "creation_time" : 100, "delivered" : true, "id" : 1, "user_id" : 10 } </code></pre> <p>Now what I want is the <code>_id</code> of all the documents per user (<code>user_id</code>) where no. of documents for a user is greater than 4. Also I want the <code>_id</code> of documents after skipping those 4 documents. So if a user has 6 documents, then I want the <code>_id</code> of last 2 documents (sorted by creation_time) so that I can archive those 2 old documents to another db.</p> <p>I am using the following query:</p> <pre><code>db.newsdb.aggregate([ { $match: { delivered: true } }, { $group: { _id: { user_id: "$user_id", creation_time: "$creation_time" } } } ]) </code></pre> <p>Now the problem is that I want to perform <code>$sort</code> and <code>$skip</code> operation on documents for each user and not on the documents of all users. So I want something like:</p> <pre><code>{ $group: { _id: { user_id: "$user_id", creation_time: "$creation_time" } }, $sort: { user_id:1, creation_time:1 }, $skip: 4 } </code></pre> <p>But it seems that mongo db doesn't support it. I am getting the following error:</p> <pre><code>Error: Printing Stack Trace at printStackTrace (src/mongo/shell/utils.js:37:7) at DBCollection.aggregate (src/mongo/shell/collection.js:897:1) at (shell):1:11 Mon Jul 1 14:47:55.762 JavaScript execution failed: aggregate failed: { "errmsg" : "exception: A pipeline stage specification object must contain exactly one field.", "code" : 16435, "ok" : 0 } at src/mongo/shell/collection.js:L898 </code></pre>
 

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