mongodb - aggregation - nested object

Question

I run the aggregate function to return the most liked items as follows:

db.users.aggregate(
    [
      { $unwind : "$favoriteItems" },
      { $group : { _id : "$favoriteItems" , number : { $sum : 1 } } },
      { $sort : { number : -1 } }
    ]
)

and this is a prototype document from my users collection

{
  "_id": "5a68a9308117670afc3522cd",
  "username": "user1",
  "favoriteItems": {
    "5a0c6711fb3aac66aafe26c6": {
      "_id": "5a0c6711fb3aac66aafe26c6",
      "name": "item1",
    },
    "5a0c6b83fd3eb67969316dd7": {
      "_id": "5a0c6b83fd3eb67969316dd7",
      "name": "item2",
    },
    "5a0c6b83fd3eb67969316de4": {
      "_id": "5a0c6b83fd3eb67969316de4",
      "name": "item3"
    }
  }
}

However, the aggregation function counts the favoriteItems as one single item and returns count per favoriteItems as opposed to elements in favoriteItems. what am I missing in the aggregate function?

s7vr · Accepted Answer · 2018-01-27 18:57:44Z

2

You can try below aggregation in 3.4. As such $unwind works on the array not on document.

So use $objectToArray to transform object to array of key value pairs.

db.users.aggregate([
  {"$addFields":{"favoriteItems":{"$objectToArray":"$favoriteItems"}}},
  {"$unwind":"$favoriteItems"},
  {"$group":{"_id":"$favoriteItems.k","number":{"$sum":1}}},
  {"$sort":{"number":-1}}
])

answered Jan 27, 2018 at 18:57

s7vr

76.2k12 gold badges118 silver badges135 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Blank Over a year ago

Thank a lot for the solution I fixed a small logical problem and it worked. Thank you so much again. I have one question: the does addFields alter the database or just the buffer used to process the documents?

s7vr Over a year ago

Np. No aggregation doesn't alter the database.

Collectives™ on Stack Overflow

mongodb - aggregation - nested object

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related