$unwind an object in aggregation framework

[*]

It is not possible to do the type of computation you are describing with the aggregation framework – and it’s not because there is no $unwind method for non-arrays. Even if the person:value objects were documents in an array, $unwind would not help.

The “group by” functionality (whether in MongoDB or in any relational database) is done on the value of a field or column. We group by value of field and sum/average/etc based on the value of another field.

Simple example is a variant of what you suggest, ratings field added to the example article collection, but not as a map from user to rating but as an array like this:

{ title : title of article", ...
  ratings: [
         { voter: "user1", score: 5 },
         { voter: "user2", score: 8 },
         { voter: "user3", score: 7 }
  ]
}

Now you can aggregate this with:

[ {$unwind: "$ratings"},
  {$group : {_id : "$ratings.voter", averageScore: {$avg:"$ratings.score"} } } 
]

But this example structured as you describe it would look like this:

{ title : title of article", ...
  ratings: {
         user1: 5,
         user2: 8,
         user3: 7
  }
}

or even this:

{ title : title of article", ...
  ratings: [
         { user1: 5 },
         { user2: 8 },
         { user3: 7 }
  ]
}

Even if you could $unwind this, there is nothing to aggregate on here. Unless you know the complete list of all possible keys (users) you cannot do much with this. [*]

An analogous relational DB schema to what you have would be:

CREATE TABLE T (
   user1: integer,
   user2: integer,
   user3: integer
   ...
);

That’s not what would be done, instead we would do this:

CREATE TABLE T (
   username: varchar(32),
   score: integer
);

and now we aggregate using SQL:

select username, avg(score) from T group by username;

There is an enhancement request for MongoDB that may allow you to do this in the aggregation framework in the future – the ability to project values to keys to vice versa. Meanwhile, there is always map/reduce.

[*] There is a complicated way to do this if you know all unique keys (you can find all unique keys with a method similar to this) but if you know all the keys you may as well just run a sequence of queries of the form db.articles.find({"ratings.user1":{$exists:true}},{_id:0,"ratings.user1":1}) for each userX which will return all their ratings and you can sum and average them simply enough rather than do a very complex projection the aggregation framework would require.