MongoDB group by with Aggregations

Question

I have a collection with items, what i want is a Aggregation where i get all item.recipes.life_skill.names grouped by their names like Heating, Cooking etc

i tried to build the Aggregation but i still have no idea

{
  _id: "recipes.life_skill.name",
  "life_skill": {
    $first: "recipes.life_skill.name"
  }
}

Here is a sample of such a item:

{
   "_id": {
      "$oid": "5f87ddc65a997f69083d663d"
   },
   "id": "756005",
   "global_id": "item--756005",
   "category": "Special Items",
   "name": "Lunar Black Stone",
   "grade": 3,
   "weight": "0.01",
   "required_level": 1,
   "url": null,
   "icon_url": null,
   "recipes": [{
       "global_id": "mrecipe--2103",
       "id": "2103",
       "life_skill": {
           "name": "Heating",
           "rank": "Beginner",
           "level": 1
       },
       "xp_amount": null,
       "url": null,
       "input_products": [{
           "type": "ITEM",
           "id": "756002",
           "name": "Frosted Black Stone",
           "quantity": 10,
           "url": null
       }, {
           "type": "ITEM",
           "id": "756003",
           "name": "Starlight Crystal",
           "quantity": 1,
           "url": null
       }],
       "output_products": [{
           "type": "ITEM",
           "id": "756005",
           "name": "Lunar Black Stone",
           "quantity": 1,
           "url": null
       }],
       "product_groups": []
   }],
   "used_in_recipes": null,
   "ship_upgrades": null,
   "achievements": null
}

varman · Accepted Answer · 2020-10-15 07:33:14Z

0

You need to perform $unwind first to destructure the array.

db.collection.aggregate([
  {
    $unwind: "$recipes"
  },
  {
    $group: {
      _id: {
        _id: "$_id",
        life_skill: "$recipes.life_skill.name"
      },
      life_skill: {
        $first: "$recipes.life_skill.name"
      }
    }
  }
])

Since you gave only one document, I used composed _id to group

_id: {
        _id: "$_id",
        life_skill: "$recipes.life_skill.name"
      }

This helps to group by each object wise. If you need overall group over all objects, simply use

_id: "$recipes.life_skill.name"

Working Mongo playground

answered Oct 15, 2020 at 7:33

varman

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