How to do "group by" in mongoengine

Question

Suppose my Schema looks like so:

class User(Document):
  username = StringField()
  password = StringField()
  category = StringField()

Imagine we have these existing categories: "avengers", "justice-leaguers", "villains", and I want to perform a "group by" query for User.objects.all() so that I can get something like this:

[
 [<User: IronMan object>, <User: Thor object>, <User: Hulk object>], 
 [<User: Superman object>,<User: Batman object>], 
 [<User: Ultron object>, <User: Joker object>, <User: LexLuthor object>]
]

Or better yet:

{
 "avengers": [<User: IronMan object>, <User: Thor object>, <User: Hulk object>], 
 "justice-leaguers": [<User: Superman object>,<User: Batman object>], 
 "villains": [<User: Ultron object>, <User: Joker object>, <User: LexLuthor object>]
}

I looked into MongoEngine's docs and have yet to find anything helpful. Thanks guys!

Answer 1

Since the OP asked for mongoengine (and thats what I needed) here is an example of this using mongoengine:

categories = User.objects.aggregate([{
    '$group': { '_id': '$category', 'username': { '$push': '$username' }}
}])

This will return a pymongo command cursor iterator.

print list(categories)

Will return:

[{ "_id": "villains", "username": ["Ultron", "Joker", "LexLuthor"]},
 { "_id": "justice-leagers", "username": ["Superman", "Batman"]},
 { "_id": "avengers", "username": ["IronMan", "Thor", "Hulk"]}]

Answer 2

Using the aggregation framework, you only need to $group documents by category:

db.User.aggregate([
  {
    $group: { _id: "$category", username: { $push: "$username" }}
  }
])

Using the $push aggregation function, you will build an array containing all the username sharing the same category.

Given you sample data:

> db.User.find({},{category:1,username:1,_id:0})
{ "category" : "avengers", "username" : "IronMan" }
{ "category" : "avengers", "username" : "Thor" }
{ "category" : "avengers", "username" : "Hulk" }
{ "category" : "justice-leagers", "username" : "Superman" }
{ "category" : "justice-leagers", "username" : "Batman" }
{ "category" : "villains", "username" : "Ultron" }
{ "category" : "villains", "username" : "Joker" }
{ "category" : "villains", "username" : "LexLuthor" }

This will produce:

{ "_id" : "villains", "username" : [ "Ultron", "Joker", "LexLuthor" ] }
{ "_id" : "justice-leagers", "username" : [ "Superman", "Batman" ] }
{ "_id" : "avengers", "username" : [ "IronMan", "Thor", "Hulk" ] }