Flask-MongoEngine & PyMongo Aggregation Query

Question

I am trying to make an aggregation query using flask-mongoengine, and from what I have read it does not sound like it is possible.

I have looked over several forum threads, e-mail chains and a few questions on Stack Overflow, but I have not found a really good example of how to implement aggregation with flask-mongoengine.

There is a comment in this question that says you have to use "raw pymongo and aggregation functionality." However, there is no examples of how that might work. I have tinkered with Python and have a basic application up using Flask framework, but delving into full fledged applications & connecting/querying to Mongo is pretty new to me.

Can someone provide an example (or link to an example) of how I might utilize my flask-mongoengine models, but query using the aggregation framework with PyMongo? Will this require two connections to MongoDB (one for PyMongo to perform the aggregation query, and a second for the regular query/insert/updating via MongoEngine)?

An example of the aggregation query I would like to perform is as follows (this query gets me exactly the information I want in the Mongo shell):

db.entry.aggregate([
    { '$group' : 
        { '_id' : { 'carrier' : '$carrierA', 'category' : '$category' }, 
          'count' : { '$sum' : 1 }
        }
    }
])

An example of the output from this query:

{ "_id" : { "carrier" : "Carrier 1", "category" : "XYZ" }, "count" : 2 }
{ "_id" : { "carrier" : "Carrier 1", "category" : "ABC" }, "count" : 4 }
{ "_id" : { "carrier" : "Carrier 2", "category" : "XYZ" }, "count" : 31 }
{ "_id" : { "carrier" : "Carrier 2", "category" : "ABC" }, "count" : 6 }

Answer 1

The class your define with Mongoengine actually has a _get_collection() method which gets the "raw" collection object as implemented in the pymongo driver.

I'm just using the name Model here as a placeholder for your actual class defined for the connection in this example:

Model._get_collection().aggregate([
    { '$group' : 
        { '_id' : { 'carrier' : '$carrierA', 'category' : '$category' }, 
          'count' : { '$sum' : 1 }
        }
    }
])

So you can always access the pymongo objects without establishing a separate connection. Mongoengine is itself build upon pymongo.

Answer 2

aggregate is available since Mongoengine 0.9. Link to the API Reference .

As there is no example whatsoever around, here is how you perform an aggregate query using aggregation framework with Mongoengine > 0.9

pipeline = [
  { '$group' : 
    { '_id' : { 'carrier' : '$carrierA', 'category' : '$category' }, 
      'count' : { '$sum' : 1 }
    }
  }]

Model.objects().aggregate(*pipeline)