I am making a query to MongoDB
db.getCollection('user_actions').aggregate([
{$match: {
type: 'play_started',
entity_id: {$ne: null}
}},
{$group: {
_id: '$entity_id',
view_count: {$sum: 1}
}},
])
and getting a list of docs with two fields:
How can I get a list of lists with two items like
[[entity_id, view_count], [entity_id, view_count], ...]
Actually there are two different way to do this, depending on your MongoDB server version.
The optimal way is in MongoDB 3.2 using the square brackets [] to directly create new array fields in the $project stage. This return an array for each group. The next stage is the another $group stage where you group your document and use the $push accumulator operator to return a two dimensional array.
db.getCollection('user_actions').aggregate([
{ "$match": {
"type": 'play_started',
"entity_id": { "$ne": null }
}},
{ "$group": {
"_id": "$entity_id",
"view_count": { "$sum": 1}
}},
{ "$project": {
"_id": 0,
"result": [ "$_id", "$view_count" ]
}},
{ "$group": {
"_id": null,
"result": { "$push": "$result" }
}}
])
From MongoDB 2.6 and prior to 3.2 you need a different approach. In order to create your array you need to use the $map operator. Because the $map "input" field must resolves to and array you need to use $literal operator to set a literal array value to input. Of course the $cond operator here returns the "entity_id" or "view_count" accordingly to the "boolean-expression".
db.getCollection('user_actions').aggregate([
{ "$match": {
"type": 'play_started',
"entity_id": { "$ne": null }
}},
{ "$group": {
"_id": "$entity_id",
"view_count": { "$sum": 1}
}},
{ "$project": {
"_id": 0,
"result": {
"$map": {
"input": { "$literal": [ "A", "B"] },
"as": "el",
"in": {
"$cond": [
{ "$eq": [ "$$el", "A" ] },
"$_id",
"$view_count"
]
}
}
}
}},
{ "$group": {
"_id": null,
"result": { "$push": "$result" }
}}
])
It worth noting that this will also work in MongoDB 2.4. If you are running MongoDB 2.2, you can use the undocumented $const operator which does the same thing.