Return multiple objects with $group(aggregation)

Question

I have a collection similar to this:

[
  {
    "_id":1,
    "name":"breakfast",
    "time":"10.00"
  },
  {
    "_id":3,
    "name":"lunch",
    "time":"12.07"
  },
  {
    "_id":2,
    "name":"breakfast",
    "time":"10.10"
  },
  {
    "_id":4,
    "name":"lunch",
    "time":"12.45"
  }
]

I want to aggregate into something like this:

{
  "breakfast":[
    {
      "_id":1,
      "name":"breakfast",
      "time":"10.00"
    },
    {
      "_id":2,
      "name":"breakfast",
      "time":"10.10"
    }
  ],
  "lunch":[
    {
      "_id":3,
      "name":"lunch",
      "time":"12.07"
    },
    {
      "_id":4,
      "name":"lunch",
      "time":"12.45"
    }
  ]
}

I have only managed to group them but I can't change the key meals to either breakfast or lunch depending on the meal.name(group name)

$group: {
    _id: { meal: '$meal.name' },
    meals: { $push: '$meal' },
}

Using the above code I have managed to produce the output below. My only challenge is changing the key meals to either breakfast or lunch as explained above in the subgroups.

{
  "meals":[
    {
      "_id":1,
      "name":"breakfast",
      "time":"10.00"
    },
    {
      "_id":2,
      "name":"breakfast",
      "time":"10.10"
    }
  ],
  "meals":[
    {
      "_id":3,
      "name":"lunch",
      "time":"12.07"
    },
    {
      "_id":4,
      "name":"lunch",
      "time":"12.45"
    }
  ]
}

Pushprajsinh Chudasama · Accepted Answer · 2019-09-11 18:44:52Z

2

Here you can have your answer .
After "grouping" to add to an array you similarly $push all that content into array by the "name" grouping key and then convert into keys of a document in a $replaceRoot with $arrayToObject:

db.collection.aggregate([
  { "$group": {
    "_id": "$name",
    "data": { "$push": "$$ROOT" }
  }},
  { "$group": {
    "_id": null,
    "data": {
      "$push": {
        "k": "$_id",
        "v": "$data"
      }
    }
  }},
  { "$replaceRoot": {
    "newRoot": { "$arrayToObject": "$data" }
  }}
])

OUTPUT

[
  {
    "breakfast": [
      {
        "_id": 1,
        "name": "breakfast",
        "time": "10.00"
      },
      {
        "_id": 2,
        "name": "breakfast",
        "time": "10.10"
      }
    ],
    "lunch": [
      {
        "_id": 3,
        "name": "lunch",
        "time": "12.07"
      },
      {
        "_id": 4,
        "name": "lunch",
        "time": "12.45"
      }
    ]
  }
]

You can check the result of above query in this LINK

edited Sep 11, 2019 at 18:44

answered Sep 11, 2019 at 18:29

Pushprajsinh Chudasama

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1 Comment

Himanshu Sharma Over a year ago

Precise and optimal approach :)

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