How to aggregate OHLC bars with mongodb?

I have 1 min OHLC bars on some market data that I imported into Mongo DB.

Each document looks like this:

{
    "_id" : ObjectId("5ac3163f31a0632c7642ca1c"),
    "Date" : "08/06/2007",
    "Time" : "15:01",
    "Open" : 1310,
    "High" : 1310.25,
    "Low" : 1309.5,
    "Close" : 1310,
    "Up" : 209,
    "Down" : 165,
    "Volume" : 0
}

I want to build a function that allows me to quickly generate X-bar intervals from this data. i.e. generate output 5-min bars, 1 hr bars, daily bars, etc... I also want to be able to filter out a data range.

I've been playing around with Mongo's aggregation functions, but I'm getting overwhelmed how I should approach this AND how I should order the pipeline operations.

Do I first group by 'Date', then sort by 'Time', then group again by $first, $last, $max and $min?

Or do I first create a new field somehow combining 'Date' and 'Time' and then proceed to the grouping?

Although don't I need to first somehow convert the "Date" and "Time" fields from string to Date field so that Mongo knows how to sort and match properly? ...but then which order would I do that in?

I'm still a newbie to MongoDB, so any advice would be appreciated.

Solution

Ok, I've come up with a solution:

db.minbars.aggregate([
   {
      $project: 
      {
         dts: 
         {
            $dateFromString: 
            {
               dateString: 
               {
                  $concat: ['$Date', '$Time']
               }
            }
         },
         Open:1, 
         High:1, 
         Low:1, 
         Close:1
      }   
   },
   {
      $match: 
      {
         dts: 
         { 
            $gte: ISODate("2016-01-01T00:00:00.000Z"), 
            $lte: ISODate("2016-12-31T00:00:00.000Z")
         }
      }
   },
   {
      $sort: { dts : 1 }
   },
   {
      $group:
      {
         _id: 
         {
            year: {$year: "$dts"},
            month: {$month: "$dts"},
            day: {$dayOfMonth: "$dts"},               
            hour: {$hour: "$dts"},
            min: 
            {
                $add: 
                [
                   {$subtract:
                   [
                      {$minute: "$dts"},
                      {$mod: [{$minute: "$dts"}, 5]}
                   ]},
                   5   
                ]
            }   
         },
         Open: {$first: "$Open"},
         High: {$max: "$High"},
         Low: {$min: "$Low"},
         Close: {$last: "$Close"}
      }
   } 
], {allowDiskUse: true})

Here's an explanation for each pipeline stage:

Project

Use 'dateFromString' from combine 'Date' and 'Time' into an ISODate object ('dts' - which stands for date timestamp). Preserve the other OHLC fields.

Match

Filter out based on a date range

Sort

Sort by the new ISODate object ('dts').

Group

Group together all those documents with the same Year, Month, Day, Hour, and 5-minute minute interval. The minute interval uses the formula: minute = minuteIn - (minuteIn % i) + i, where i=minute interval. I'm adding 'i' so that minutes 00, 01, 02, 03, and 04 are aggregated to the next 05 minute interval (and not the preceding 00 minute interval). NOTE: if you want 1-hr, 4-hr, Daily bars, etc...then you need to adjust the _id section accordingly.

NOTE: I'm using {allowDiskUse: true} here because at one point I ran into the Memory constraints at the Sort stage.

Maybe someone can come up with a simpler way to do this?

UPDATE:

As I noted in 4) above, I mentioned I was adding "i" (minute interval) to the resulting minute. However, when I did this I ended up with a '60' minute interval showing up in the output. You should only have 0, 5, 10, 15, ...55 minute bars and there should NOT be a 60-minute bar. So this was NOT correct.

Also, if you compare against a trading platform (i.e. Thinkorswim) you can see that the standard practice is to use preceding 5-minute interval for the timestamp of the bar. For example, the 5-minute bar 9:25 represents the aggregation of these minute bars: 9:25, 9:26, 9:27, 9:28, 9:29.