Group all objects using aggregation framework

Question

I have a collection of objects in MongoDB that looks like this

    { "event_type": "event1", "created_at": some_date, ...more_fields... },
    { "event_type": "event2", "created_at": some_date, ...other_fields... },
    { "event_type": "event1", "created_at": some_date, ...more_fields... }

Now I want to have the above data with all fields grouped by some field (which might not be present in all objects) and order it by the highest created_at date. I have tried to do this using the Aggregation Framework with the following query:

  collection.aggregate([
    { "$group" => {
        :_id       => "$somefield",
        :last_time => { "$max"  => "$created_at" },
        :events    => { "$push" => { ... } }
      }
    },
    { "$match" => { "_id" => { "$ne" => nil } } },
    { "$sort" => { "last_time" => -1 } }
  ])

The problem I'm facing is related to the line

 :events    => { "$push" => { ... } }

If I put in some specific fields then it works, but I don't know which exact fields the collection contains. But rather I want the whole object returned like this:

    {
        "event_type": "event1", "last_time": some_date, "events": [
            { "event_type": "event1", "created_at": some_date, ...more_fields... },
            { "event_type": "event1", "created_at": some_date, ...more_fields... },
            ...
        ]
    }

cirrus · Accepted Answer · 2012-10-11 14:50:56Z

1

Assuming you want to push ALL the fields in your object, without knowing exactly what those fields are, couldn't you just wrap them in a known field parent and push that?

{ events: {
    "event_type": "event1", "created_at": some_date, ...more_fields... 
  }
},

Then you just need to push "events".

answered Oct 11, 2012 at 14:50

cirrus

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1 Comment

derfred Over a year ago

That would work, but I would rather not have to change the shape of the data. I ended up doing it with a simple group call and sorting on the client.

sambomartin · Accepted Answer · 2012-10-11 14:49:39Z

0

if you do the match first, you'll be able to use the index (if one exists on the "somefield")

could it be you need to project the sub doc you want to create an array of for each aggregated value? I haven't tested this, but might will hopefully point you in right direction?

something like?

db.collection.aggregate( 
{ $match : { "some_field" : { $ne:null} } },  
{ $project : 
    { 
        event_type :"event_type", 
        created_at: "created_at",
        event: 
        {
            another_att: "attrib1",
            another_att2: "attrib2"
        }
     },
{ $group : 
    { 
        _id: "$event_type",
        lasttime : { $max: "$created_at"},
        theevents: { $addToSet : "$event" }
    }
 })

answered Oct 11, 2012 at 14:49

sambomartin

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Comments

derfred · Accepted Answer · 2012-10-11 17:14:18Z

0

This problem is solved more easily using the group function:

options = {
  key:      "field_to_group_by",
  cond:     { ..some filter criteria.. },
  initial:  { :events  => [] },
  reduce:   "function(obj, agg) { agg.events.push(obj); }"
}
collection.group(options)

This doesn't return the items in the order I want, but sorting can be done on the client.

answered Oct 11, 2012 at 17:14

derfred

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1 Comment

cirrus Over a year ago

can't you just add .Sort() on group() ?

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