How to merge objects of array in MongoDB

Question

I'm trying to merge objects of an array in the aggregation pipeline. Here is MongoDB Document.

{
    "category" : [
        {
            "id" : ObjectId("60ed3d041436f1310401b1b2"),
            "name" : "cat 1",
            "services" : [
                {
                    "_id" : ObjectId("607ab64bc520c2587045694b"),
                    "name" : "service 1",
        
                }
            ]
        }
    ]
},

{
    "category" : [
        {
            "id" : ObjectId("60ed3cff1436f1310401b1b1"),
            "name" : "cat 2",
            "services" : [
                {
                    "_id" : ObjectId("5ffd5d4bc4967a1b10bf5022"),
                    "name" : "service 2",
                
                }
            ]
        },
        {
            "id" : ObjectId("60ed3cff1436f1310401b1b1"),
            "name" : "cat 2",
            "services" : [
                {
                    "_id" : ObjectId("6077e777ad481470d42c4e41"),
                    "name" : "service 3",
                    
                }
            ]
        }
    ]
}

Expected Output I need to merge the category array into one and merge the service array based on the category

     [
        {
            "id" : ObjectId("60ed3d041436f1310401b1b2"),
            "name" : "cat 1",
            "services" : [
                {
                    "_id" : ObjectId("607ab64bc520c2587045694b"),
                    "name" : "service 1",
        
                }
            ]
        },
        {
            "id" : ObjectId("60ed3cff1436f1310401b1b1"),
            "name" : "cat 2",
            "services" : [
                {
                    "_id" : ObjectId("5ffd5d4bc4967a1b10bf5022"),
                    "name" : "service 2",
                
                },
                {
                    "_id" : ObjectId("6077e777ad481470d42c4e41"),
                    "name" : "service 3",
                    
                }
            ]
        },
    ]

How to achieve this ?.

Not possible! You cannot have just an array inside an object. — Wernfried Domscheit
– Wernfried Domscheit, Commented Sep 20, 2021 at 13:40

Wernfried Domscheit · Accepted Answer · 2021-09-20 14:34:24Z

2

Try this one:

db.collection.aggregate([
  { $unwind: "$category" },
  {
    $group: {
      _id: {
        _id: "$_id",
        name: "$category.name"
      },
      services: { $push: { $first: "$category.services" } }
    }
  },
  {
    $project: {
      _id: "$_id._id",
      name: "$_id.name",
      services: 1
    }
  }
])

See Mongo Playground

answered Sep 20, 2021 at 14:34

Wernfried Domscheit

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6 Comments

Sufail Kalathil Over a year ago

I got an error .MongoError: Unrecognized expression '$first'

Wernfried Domscheit Over a year ago

Which mongo version do you use? Try { $arrayElemAt: [ "$category.services", 0 ] } instead.

Sufail Kalathil Over a year ago

mongoDB version "4.2.2"

Sufail Kalathil Over a year ago

Could you please explain the complete code by using $arrayElemAt

Wernfried Domscheit Over a year ago

Should be obvious: { $first: "$category.services" } -> { $arrayElemAt: [ "$category.services", 0 ] }

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