Get Distinct list of two properties using MongoDB 2.4 - mongodb

Question

I have an article collection:
{
_id: 9999,
authorId: 12345,
coAuthors: [23456,34567],
title: 'My Article'
},
{
_id: 10000,
authorId: 78910,
title: 'My Second Article'
}
I'm trying to figure out how to get a list of distinct author and co-author ids out of the database. I have tried push, concat, and addToSet, but can't seem to find the right combination. I'm on 2.4.6 so I don't have access to setUnion.

Answer 1

Whilst $setUnion would be the "ideal" way to do this, there is another way that basically involved "switching" between a "type" to alternate which field is picked:
db.collection.aggregate([
{ "$project": {
"authorId": 1,
"coAuthors": { "$ifNull": [ "$coAuthors", [null] ] },
"type": { "$const": [ true,false ] }
}},
{ "$unwind": "$coAuthors" },
{ "$unwind": "$type" },
{ "$group": {
"_id": {
"$cond": [
"$type",
"$authorId",
"$coAuthors"
]
}
}},
{ "$match": { "_id": { "$ne": null } } }
])
And that is it. You may know the $const operation as the $literal operator from MongoDB 2.6. It has always been there, but was only documented and given an "alias" at the 2.6 release.
Of course the $unwind operations in both cases produce more "copies" of the data, but this is grouping for "distinct" values so it does not matter. Just depending on the true/false alternating value for the projected "type" field ( once unwound ) you just pick the field alternately.
Also this little mapReduce does much the same thing:
db.collection.mapReduce(
function() {
emit(this.authorId,null);
if ( this.hasOwnProperty("coAuthors"))
this.coAuthors.forEach(function(id) {
emit(id,null);
});
},
function(key,values) {
return null;
},
{ "out": { "inline": 1 } }
)
For the record, $setUnion is of course a lot cleaner and more performant:
db.collection.aggregate([
{ "$project": {
"combined": {
"$setUnion": [
{ "$map": {
"input": ["A"],
"as": "el",
"in": "$authorId"
}},
{ "$ifNull": [ "$coAuthors", [] ] }
]
}
}},
{ "$unwind": "$combined" },
{ "$group": {
"_id": "$combined"
}}
])
So there the only real concerns are converting the singular "authorId" to an array via $map and feeding an empty array where the "coAuthors" field is not present in the document.
Both output the same distinct values from the sample documents:
{ "_id" : 78910 }
{ "_id" : 23456 }
{ "_id" : 34567 }
{ "_id" : 12345 }