Mongo average aggregation query with no group

Question

I am trying to get the average of a whole field using the aggregation framework in Mongo. However i can't seem to find any example that uses it without a group parameter.

I have the following document structure:

 {
      "_id" : ObjectId("5352703b61d2739b2ea44e4d"),
      "Semana" : "2014-02-23 - 2014-03-01",
      "bolsaDeValores" : "7",
      "bvc" : "8",
      "dollar" : "76",
      "ecopetrol" : "51",
      "dollarPrice" : "18"
 }

Basically what i want to do is get the average value of the bvc field, and any other numeric one, for the whole collection in the fastest possible way (without using MapReduce as it is less efficient than the Aggregation Framework).

I have tried to group on a greater than zero basis as well but to no avail:

db.EvaluatedSentiments.aggregate([
    { "$group": { 
        "bvc" : {"$gt:0"}
        }, 
        {
            "bvc" : { "$avg" : "$bvc"}
        }
    }
])

I appreciate any help you could provide.

References: Mongo aggregation manual

Answer 1

First of all store numerical values as numbers. Afterwards you can use a simple statement to calculate the average:

db.collection.aggregate([{ 
  "$group": {
    "_id": null, 
    "avg_bvc": { "$avg": "$bvc" } 
  } 
}])

You can simply use more $avg aggregation operators to get averages for your other numeric fields:

db.collection.aggregate([{ 
  "$group": {
    "_id": null, 
    "avg_bvc": { "$avg": "$bvc" }, 
    "avg_dollar": { "$avg": "$dollar" } 
  } 
}])

Answer 2

So if your data actually was numeric which is it not and your intention is to exclude the documents that have a "greater than zero" value then you include a $match statement in your aggregation pipeline in order to "filter" out these documents:

db.EvaluatedSentiments.aggregate([
    { "$match": {
        "bvc": { "$gt": 0 }
    }},
    { "$group": {
        "_id": null,
        "bvc": { "$avg": "$bvc" }
    }}
])