MongoDB projection into array - mongodb

Question

The below document has the dob of student and its parent's dob.
{
"_id" : ObjectId("56a31573a3b1f89cb895abd3"),
"dob" : {
"isodate" : ISODate("1996-01-21T18:30:00.000+0000")
},
"parent" : [
{
"dob" : {
"isodate" : ISODate("1956-07-21T18:30:00.000+0000")
},
"type" : "father"
},
{
"dob" : {
"isodate" : ISODate("1958-11-01T18:30:00.000+0000")
},
"type" : "mother"
}
]
}
In one of the application use case, it is better to receive output in the below format
{
"_id" : ObjectId("56a31573a3b1f89cb895abd3"),
"dob" : {
"isodate" : ISODate("1996-01-21T18:30:00.000+0000")
},
"type" : "student"
},
{
"_id" : ObjectId("56a31573a3b1f89cb895abd3"),
"dob" : {
"isodate" : ISODate("1956-07-21T18:30:00.000+0000")
},
"type" : "father"
},
{
"_id" : ObjectId("56a31573a3b1f89cb895abd3"),
"dob" : {
"isodate" : ISODate("1958-11-01T18:30:00.000+0000")
},
"type" : "mother"
}
The approach is to $project the fields into array and then $unwind that array. However, projection doesn't allow me to create array.
I believe $group and its associated aggregation cannot be used as my operations are on the same document in the pipeline.
Is this possible?
Note - i have the flexibility to change the document design as well.

Answer 1

For Mongo 3.0
Here I have included a [null] array which gives me the option to insert array in projection using a combination of $setDiffernce and $cond. The output of this is given to $setUnion with $parent array.
db.p1.aggregate(
{ "$project": {
"allVal": {
'$setUnion': [
{"$setDifference": [
{ "$map": {
"input": [null],
"as": "type",
"in": { "$cond": [
{"$eq": ["$$type", null]},
{dob:"$dob", type:{$literal:'student'}},
null
]}
}},
[null]
]}
,
'$parent'
]
}
}},
{$unwind : '$allVal'}
)
For mongo 3.2
Feels heaven as I have avoided $setDifference and $literal hack adjustments.
db.p1.aggregate([
{
$project:{
parent : 1,
type: {$literal : 'student'},
'dob.isodate' : 1
}
},
{
$project:{
allValues: { $setUnion: [ [{dob:"$dob", type:'$type'}], "$parent" ] }
}
},
{
$unwind : '$allValues'
}
])
In the first projection, I am adding a new field called type
In the 2nd projection, I am creating a new array with 2 different nodes of the same document.
Currently this solution works for Mongo 3.2