Perform union in mongoDB - mongodb

Question

I'm wondering how to perform a kind of union in an aggregate in MongoDB. Let's imaging the following document in a collection (the structure is for the sake of the example) :
{
linkedIn: {
people : [
{
name : 'Fred'
},
{
name : 'Matilda'
}
]
},
twitter: {
people : [
{
name : 'Hanna'
},
{
name : 'Walter'
}
]
}
}
How to make an aggregate that returns the union of the people in twitter and linkedIn ?
{
{ name :'Fred', source : 'LinkedIn'},
{ name :'Matilda', source : 'LinkedIn'},
{ name :'Hanna', source : 'Twitter'},
{ name :'Walter', source : 'Twitter'},
}

Answer 1

There are a couple of approaches to this that you can use the aggregate method for
db.collection.aggregate([
// Assign an array of constants to each document
{ "$project": {
"linkedIn": 1,
"twitter": 1,
"source": { "$cond": [1, ["linkedIn", "twitter"],0 ] }
}},
// Unwind the array
{ "$unwind": "$source" },
// Conditionally push the fields based on the matching constant
{ "$group": {
"_id": "$_id",
"data": { "$push": {
"$cond": [
{ "$eq": [ "$source", "linkedIn" ] },
{ "source": "$source", "people": "$linkedIn.people" },
{ "source": "$source", "people": "$twitter.people" }
]
}}
}},
// Unwind that array
{ "$unwind": "$data" },
// Unwind the underlying people array
{ "$unwind": "$data.people" },
// Project the required fields
{ "$project": {
"_id": 0,
"name": "$data.people.name",
"source": "$data.source"
}}
])
Or with a different approach using some operators from MongoDB 2.6:
db.people.aggregate([
// Unwind the "linkedIn" people
{ "$unwind": "$linkedIn.people" },
// Tag their source and re-group the array
{ "$group": {
"_id": "$_id",
"linkedIn": { "$push": {
"name": "$linkedIn.people.name",
"source": { "$literal": "linkedIn" }
}},
"twitter": { "$first": "$twitter" }
}},
// Unwind the "twitter" people
{ "$unwind": "$twitter.people" },
// Tag their source and re-group the array
{ "$group": {
"_id": "$_id",
"linkedIn": { "$first": "$linkedIn" },
"twitter": { "$push": {
"name": "$twitter.people.name",
"source": { "$literal": "twitter" }
}}
}},
// Merge the sets with "$setUnion"
{ "$project": {
"data": { "$setUnion": [ "$twitter", "$linkedIn" ] }
}},
// Unwind the union array
{ "$unwind": "$data" },
// Project the fields
{ "$project": {
"_id": 0,
"name": "$data.name",
"source": "$data.source"
}}
])
And of course if you simply did not care what the source was:
db.collection.aggregate([
// Union the two arrays
{ "$project": {
"data": { "$setUnion": [
"$linkedIn.people",
"$twitter.people"
]}
}},
// Unwind the union array
{ "$unwind": "$data" },
// Project the fields
{ "$project": {
"_id": 0,
"name": "$data.name",
}}
])

Answer 2

Not sure if using aggregate is recommended over a map-reduce for that kind of operation but the following is doing what you're asking for (dunno if $const can be used with no issue at all in the .aggregate() function) :
aggregate([
{ $project: { linkedIn: '$linkedIn', twitter: '$twitter', idx: { $const: [0,1] }}},
{ $unwind: '$idx' },
{ $group: { _id : '$_id', data: { $push: { $cond:[ {$eq:['$idx', 0]}, { source: {$const: 'LinkedIn'}, people: '$linkedIn.people' } , { source: {$const: 'Twitter'}, people: '$twitter.people' } ] }}}},
{ $unwind: '$data'},
{ $unwind: '$data.people'},
{ $project: { _id: 0, name: '$data.people.name', source: '$data.source' }}
])