Query to get last X minutes data with Mongodb

Question

I'm trying to query my db that have this document format:

{   "_id" : ObjectId("520b8b3f8bd94741bf006033"),   "value" : 0.664,   "timestamp" : ISODate("2013-08-14T13:48:35Z"),   "cr" : ISODate("2013-08-14T13:50:55.834Z") } 

I can get the last records from a datetime with this query:

db.mycol.find({timestamp:{$gt: ISODate("2013-08-14T13:48:00Z")}}).sort({x:1}); 

But I'm trying to get a set with the value fields and timestamps from 18 minutes ago.

Answer 1

For the 18 minutes part, that's not really about MongoDB, but about JavaScript and what's available in the mongo shell:

query = {     timestamp: { // 18 minutes ago (from now)         $gt: new Date(ISODate().getTime() - 1000 * 60 * 18)     } } 

Works in the mongo shell, but using Mongo drivers for other languages would be really different.

To "project" over a smaller schema with both values and timestamps:

projection = {     _id: 0,     value: 1,     timestamp: 1, } 

Applying both:

db.mycol.find(query, projection).sort({timestamp: 1}); 

Well, that's still not a "set" since there might be duplicates. To get rid of them you can use the $group from the aggregation framework:

db.mycol.aggregate([     {$match: query},     {$group: {         _id: {             value: "$value",             timestamp: "$timestamp",         }     }},     {$project: {         value: "$_id.value",         timestamp: "$_id.timestamp",     }},     {$sort: {timestamp: 1}}, ])