How to count array elements in mongo

Question

I know there are lots of topics talking about that out there but I can't figure out what goes wrong in my implementation.

I have the followings documents:

{
  "_id" : ObjectId("510a353929e16756d5000009"),
  "skills" : [
    [
      "UserEmbed::Author::Skills::Copywriting",
      "UserEmbed::Author::Skills::Proofreading",
      "UserEmbed::Author::Skills::Proofreading",
      "UserEmbed::Author::Skills::Translation",
      "UserEmbed::Author::Skills::Translation",
      "UserEmbed::Author::Skills::Translation"
    ]
  ]
}

I would like something like this:

{
  "UserEmbed::Author::Skills::Copywriting": 1,    
  "UserEmbed::Author::Skills::Proofreading": 2,
  "UserEmbed::Author::Skills::Translation": 3
}

Here is what I have (first $group is to get the document above from my original document structure):

aggregate([ { $group: { _id: "$_id", skills: { $addToSet : "$author_profile.skills._type" } } }, { $unwind : "$skills" }, { $group : { _id : "$skills", count: { $sum : 1 } } }])

Which returns something like this (with other documents):

{
  "_id" : [
    "UserEmbed::Author::Skills::Copywriting",
    "UserEmbed::Author::Skills::Copywriting",
    "UserEmbed::Author::Skills::Copywriting",
    "UserEmbed::Author::Skills::Translation",
    "UserEmbed::Author::Skills::Translation",
    "UserEmbed::Author::Skills::Translation"
  ],
  "count" : 1
}

It seems that the $group is not working properly. Did I misunderstand something ?

WiredPrairie · Accepted Answer · 2013-07-09 12:43:54Z

6

Given your document contains an array of arrays, you'd need to introduce a second $unwind for $skills:

db.so.aggregate([
    { $group: { _id: "$_id", 
        skills: { $addToSet : "$author_profile.skills._type" }}},
    { $unwind : "$skills" }, 
    { $unwind: "$skills" },
    { $group : { _id : "$skills", count: { $sum : 1 } } }])

Produces:

"result" : [
        {
            "_id" : "UserEmbed::Author::Skills::Translation",
            "count" : 3
        },
        {
            "_id" : "UserEmbed::Author::Skills::Proofreading",
            "count" : 2
        },
        {
            "_id" : "UserEmbed::Author::Skills::Copywriting",
            "count" : 1
        }
    ],
"ok" : 1

answered Jul 9, 2013 at 12:43

WiredPrairie

59.9k18 gold badges120 silver badges149 bronze badges

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

3 Comments

Pierre-Louis Gottfrois Over a year ago

Do you know how I could group the results by the first _id ?

WiredPrairie Over a year ago

I don't understand what you mean ... group by which _id?

Pierre-Louis Gottfrois Over a year ago

I figured it out, wanted to group by the initial _id. Did that _id: { user_id: "$_id", type: "$skills" }

Collectives™ on Stack Overflow

How to count array elements in mongo

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related