How to get (or aggregate) distinct keys of array in MongoDB

If you do not know the keys or cannot make a reasonable educated guess then you are basically stuck from going any further with the aggregation framework. You could supply "all of the keys" for consideration, but I supect your acutal data looks more like this:

{ "data": [{ "film": 10 }, { "televsion": 5 },{ "boardGames": 1 }] }

So there would be little point here findin out all the "key names" and then throwing that at an aggregation statement.

For the record though, "this is why you do not structure your data storage like this". Information like "film" here should not be used as a "key" name, because it is useful "data" that could be searched upon and most importantly "indexed" in a database system.

So your data should really look like this:

{ 
    "data": [
        { "type": "film", "value": 10 },
        { "type": "televsion", "valule": 5 },
        { "type": "boardGames", "value": 1 }
    ]
}

Then the aggregation statement is simple, as are many other things:

db.collection.aggregate([
    { "$unwind": "$data" },
    { "$group": {
        "_id": null,
        "sum": { "$sum": "$data.value" },
        "avg": { "$avg": "$data.value" }
    }}
])

But since the key names are constantly changing in documents and do not have a uniform structure, then you need JavaScript processing on the server to traverse the keys, and that meand mapReduce:

db.collection.mapReduce(
    function() {
        this.data.forEach(function(data) {
            Object.keys(data).forEach(function(key) {
                emit(null,data[key]); // emit the value regardless of key name
            });
        });
    },
    function(key,values) {
        return Array.sum(values);     // Just summing for example
    },
    { "out": { "inline": 1 } }
)

And of course the JavaScript execution here will work much more slowly than the native coded operators available to the aggregation framework.

So this should be an abject lesson as to why you don not use "data" as "key names" when storing data in a database. The aggregation framework works with standard structres and is fast, falling back to JavaScript processing is more flexible, but the cost is mostly in speed and other features.