how to get this query in mongoDB - mongodb

Question

I have this collection...
> db.banks.find().pretty()
{
"_id" : ObjectId("54f37cbb44aec3b01b7db8f4"),
"name" : "A",
"branches" : [
{
"branch_id" : 8561,
"name" : "X",
},
{
"branch_id" : 8576,
"name" : "Y",
}
]
}
{
"_id" : ObjectId("54f37cbb44aec3b01b7db8f5"),
"name" : "B",
"branches" : [
{
"branch_id" : 3238,
"name" : "Z",
}
]
}
with this command :
db.banks.aggregate({$project{"branches.name":1,"_id":0}});
get this result :
{ "branches" : { { "name" : "X" }, { "name" : "Y" } } }
{ "branches" : { { "name" : "Z" } } }
but; how I get this result?
(In fact, one object and without "branches".)
{{"name" : "X"}, {"name" : "Y"}, {"name" : "Z"}}
very thanks...

Answer 1

One way you could go about this is to do an $unwind first in the aggregation pipeline to get a deconstructed array with a document for each element and then group by the array element $branches.name:
db.banks.aggregate([
{ $unwind: '$branches'},
{
$group: {
_id: {
name: '$branches.name'
}
}
},
{
$project: {
_id: 0,
name: '$_id.name'
}
},
{ $sort : { "name" : 1 } }
])
Outputs:
{
"result" : [
{
"name" : "X"
},
{
"name" : "Y"
},
{
"name" : "Z"
}
],
"ok" : 1
}