Plain query in mongoDB - mongodb

Question

I have a collection called constructora that has the following structure:
{
"_id" : A,
"edificio" : [
{
"_id": X,
"a" : 48,
"b" : 59
},
{
"_id": Y,
"a" : 8,
"b" : 5
},
{
"_id": Z,
"a" : 0,
"b" : -1
},
...
]
}
So, I want to make a query that returns, for each sub document (edificio) his parent's _id. An example:
{
"_id" : X,
"a" : 48,
"b" : 59
"id_constructora" : A
}
{
"_id" : Y,
"a" : 8,
"b" : 5
"id_constructora" : A
}
{
"_id" : Z,
"a" : 0,
"b" : -1
"id_constructora" : A
}
How can I do that?
EDIT
Now I'm trying using aggregate, and grouping my query by "edificio_id", so for each document in edificio I can get my desired output:
db.constructora.aggregate(
[
{ $project : { "_id" : 1, "edificio._id" : 1 } },
{ $group : { _id : "$edificio._id" } }
]
).pretty();
But it doesn't work. The output is:
...
{
"_id" : [
ObjectId("613339376430333562373466"),
ObjectId("663736363935393066656236"),
ObjectId("313933613036363364633832"),
ObjectId("653135313831633638336436")
]
}
{
"_id" : [
ObjectId("643531326231663739626465"),
ObjectId("343231386237333365356461"),
ObjectId("373461303864636138393263"),
ObjectId("386433623966653737343962"),
ObjectId("303863633366376431363335"),
ObjectId("663833343161643639376161"),
ObjectId("383833363836663532633733"),
ObjectId("396330313961353137333166"),
ObjectId("646535366662363364613837"),
ObjectId("633937613032656436653965")
]
}

Answer 1

You can use $unwind to break the embedded array into embedded docs, $addFields to rename and add the _id into the embedded doc followed by $replaceRoot to promote the embedded document to the top level in 3.4 mongo server.
db.constructora.aggregate([
{$unwind:"$edificio"},
{$addFields:{"edificio.id_constructora":"$_id"}},
{$replaceRoot: {newRoot: "$edificio"}}
])
More info here https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/#replaceroot-with-an-array-element