how to get this query in 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...

chridam · Accepted Answer · 2015-03-02 12:29:52Z

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 }

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