Get key, value aggregation of values within an array using MongoDB?

Question

I have a MongoDB schema for blog posts that looks like:

{
  title: String,
  date: Date,
  description: String,
  tags: [String]
}

So, for example, a given blog post might have a document that looks like:

{
  title: 'My blog post about MongoDB',
  date: '2020-07-06T07:00:00.000+00:00',
  description: 'A blog about Mongo',
  tags: ['MongoDB', 'Javascript']
}

I'm trying to implement a query across my db that returns a mapping of each tag and a count of how many times that tag appears in the collection. So, for example if my collection has three posts (A, B, and C), where:

A.tags = ['MongoDB', 'Random']
B.tags = ['MongoDB', 'React']
C.tags = ['Nature', 'Random']

The query would return:

[
   { 'MongoDB', 2 },
   { 'Random', 2 },
   { 'React', 1 },
   { 'Nature', 1 },
   { 'Coding', 1 },
]

Is there a way to do so using some sort of aggregation pipeline or something similar?

Thanks!

Does this answer your question? MongoDB group by array inner-elements — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Jun 12, 2020 at 18:45

harshit kohli · Accepted Answer · 2020-06-12 19:06:08Z

1

Yes it's possible

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

PS: result format will be different but it will give the correct result you wanted

answered Jun 12, 2020 at 19:06

harshit kohli

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2 Comments

whoami - fakeFaceTrueSoul Over a year ago

that's exactly the same what's given in link above, Please do not add answers for duplicate questions unless OP gets back asking for more help ! It just make redundant questions, try to mark questions as duplicates..

harshit kohli Over a year ago

@whoami i somehow missed your comment! otherwise i would have done the same!

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