Extract list of data from collections-CodePudding

I have a list of collection like the following in mongodb

   { _id: ObjectId("6087feeef467a4320883daf3"),
     name: 'group 1',
     admins: ['adam', 'ken']
   }

   { _id: ObjectId("2087feeef467a4320883daf3"),
     name: 'group 2',
     admins: ['rebecca']
   }

   { _id: ObjectId("9987feeef467a4320883daf3"),
     name: 'group 3',
     admins: []
   }

I need to extract out all the admins, the end result would be something like this:

[
  'admin',
  'ken',
  'rebecca'
]

How can i do that, i stuck at this part:

db.data.find({ admins: { $exists: true, $not: {$size: 0} } })

This will show all collections, but i just need the list

CodePudding user response：

The proper way of doing so using single query is unwinding the array of admins and adding them to a new set.

db.collection.aggregate([
  {
    $unwind: "$admins"
  },
  {
    $group: {
      _id: null,
      adminsSet: {
        $addToSet: "$admins"
      }
    }
  }
])

The end result will be the following so you could extract your flattened array:

[
  {
    "_id": null,
    "adminsSet": [
      "ken",
      "rebecca",
      "adam"
    ]
  }
]

Mongo playground ref: