Using $project to return an array - mongodb

Question

I have a collection with documents which look like this:
{
"campaignType" : 1,
"allowAccessControl" : true,
"userId" : "108028399"
}
I'd like to query this collection using aggregation framework and have a result which looks like this:
{
"campaignType" : ["APPLICATION"],
"allowAccessControl" : "true",
"userId" : "108028399",
}
You will notice that:
campaignType field becomes and array
the numeric value was mapped to a string
Can that be done using aggregation framework?
I tried looking at $addToSet and $push but had no luck.
Please help.
Thanks

Answer 1

In either case here it is th $cond operator from the aggregation framework that is your friend. It is a "ternary" operator, which means it evaluates a condition for true|false and then returns the result based on that evaluation.
So for modern versions from MongoDB 2.6 and upwards you can $project with usage of the $map operator to construct the array:
db.campaign.aggregate([
{ "$project": {
"campaignType": {
"$map": {
"input": { "$literal": [1] },
"as": "el",
"in": {
"$cond": [
{ "$eq": [ "$campaignType", 1 ] },
"APPLICATION",
false
]
}
}
},
"allowAcessControl" : 1,
"userId": 1
}}
])
Or generally in most versions you can simply use the $push operator in a $group pipeline stage:
db.campaign.aggregate([
{ "$group": {
"_id": "$_id",
"campaignType": {
"$push": {
"$cond": [
{ "$eq": [ "$campaignType", 1 ] },
"APPLICATION",
false
]
}
},
"allowAccessControl": { "$first": "$allowAccessControl" },
"userId": { "first": "$userId" }
}}
])
But the general concept if that you use "nested" expressions with the $cond operator in order to "test" and return some value that matches your "mapping" condition and do that with another operator that allows you to produce an array.