Groupby Array elements in Mongodb Query

Question

I have an array element in my DB and I want to group By and calculate the number of repetitions of different elements in this array. assume following collection:

{
   _id: ObjectId(7df78ad8902c)
   title: 'MongoDB Overview', 
   tags: ['SQL', 'database', 'NoSQL'],
},
{
   _id: ObjectId(7df78ad8902d)
   title: 'NoSQL Overview', 
   tags: ['mongodb', 'database', 'PHP'],
}

I want to calculate the number of repetitions of different elements in tags field such as mongodb, database, noSQL in this collection. How can I solve this problem in mongo?

Takis · Accepted Answer · 2021-10-02 18:05:32Z

1

Query

counts the repetitions in all the collection
unwind to make each tag in seperate document
group by tag, and count the repetitions

Test code here

db.collection.aggregate([
  {
    "$unwind": {
      "path": "$tags"
    }
  },
  {
    "$group": {
      "_id": "$tags",
      "count": {
        "$sum": 1
      }
    }
  },
  {
    "$project": {
      "_id": 0,
      "tag": "$_id",
      "count": 1
    }
  }
])

answered Oct 2, 2021 at 18:05

Takis

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7 Comments

Ali Over a year ago

Wow it works. thank you so much. It is a little complicated for beginner :)

Ali Over a year ago

can you give me a good reference to exercise these things ?

Takis Over a year ago

no its very easy, $unwind looks scary at start, but its not, read mongodb documentation its very good

Takis Over a year ago

here its the reference with all the operators with examples, see the pipeline stage operators to find the $unwind

Takis Over a year ago

create a new question if you can, with sample data in, and expected data out. Seems like you will need $hour to get the hour from the date, and then group by tag + hour, 2 fields in group.

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