MongoDB group by values in an array field

Question

I have my asset documents in the below format.

db.asset.find({}).limit(1).pretty()
{
    "_id" : ObjectId("54e650a10364a65f62c0df4a"),
    "_class" : "com.model.core.Asset",
    "displayName" : "Bingo Rhymes For Children + More 3D Animation Nursery Rhymes & Kids' Songs",
    "assetType" : "VIDEO",
    "active" : true,
    "originalUrl" : "https://www.youtube.com/watch?v=j-tdVvvXn9k&feature=youtube_gdata",
    "groupIds" : [ ],
    "folderIds" : [
        "54e6507b0364a65f62c0df47",
        "54e6507b0364a65f62c0df48"
    ]
}

As you can see each asset can have a collection of folderId to which it is associated with. If I want to find the folderIds along with the associated assets how does the mongo aggregate query will look like? Essentially I want to group the assets by folderId.

It is unclear what you are asking here. What are you trying to do? what is the expected result? — Sede
– Sede, Commented Oct 14, 2015 at 20:04
@user3100115 Essentially I want to group the assets by folderId. — Srini Kandula
– Srini Kandula, Commented Oct 14, 2015 at 20:19

sergiuz · Accepted Answer · 2015-10-14 20:47:26Z

7

You first need to unwind by the folderIds field, than group by _id and push the asset _id into a list assets_id.

db.asset.aggregate([{$unwind:"$folderIds"},  {$group:{_id: "$folderIds",assets:{$push: {assets_id:"$_id"}}}}])

edited Oct 14, 2015 at 20:47

answered Oct 14, 2015 at 20:28

sergiuz

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3 Comments

Srini Kandula Over a year ago

I think this gets me one-on-one association of the folderId vs assetId. Here is the results of the query you suggested. db.asset.aggregate([{$unwind:"$folderIds"}, {$group:{_id:{"folderIds":"$folderIds","asset_id":"$_id"}}}]) { "_id" : { "folderIds" : "54f8a2ed0364edbff0152df3", "asset_id" : ObjectId("55a53cfd93861c7d709af3c7") } } { "_id" : { "folderIds" : "54f8a2ed0364edbff0152df3", "asset_id" : ObjectId("552ff98d9386ca008fe2464c") } }. Can I get the assetIds grouped together for each folderId.

Srini Kandula Over a year ago

Thank you but the new query doesn't return any results.

Srini Kandula Over a year ago

Thank you. This works. db.asset.aggregate([{$unwind:"$folderIds"}, {$group:{_id: "$folderIds",assets:{$push: {assets_id:"$_id"}}}}])

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