Looping through array to count in mongodb/mongoose

Question

I have a user schema that contains a value called amputationInfo:

amputationInfo: [
      {
        type: String,
      },
    ],

Here is an example of what that might look like in the database:

amputationInfo: [
 "Double Symes/Boyd", 
  "Single Above-Elbow"
]

I have a review Schema that allows a user to leave a review, it contains a reference to the user who left it:

user: {
  type: mongoose.Schema.ObjectId,
  ref: 'User',
  require: [true, 'Each review must have an associated user!'],
},

When a user leaves a review, I want to create an aggregate function that looks up the user on the review, finds their amputationInfo, loops through the array and adds up the total amount of users that contain "Double Symes/Boyd", "Single Above-Elbow"

So if we have 3 users and their amputationInfo is as follows:

 amputationInfo: [
  "Double Symes/Boyd", 
  "Single Above-Elbow"
 ]

amputationInfo: [ 
  "Single Above-Elbow"
]

amputationInfo: []

The return from the aggregate function will count each term and add one to the corresponding value and look something like this:

[
 {
    doubleSymesBoyd: 1,
    singleAboveElbow: 2
  }
]

Here is what I have tried, but I just don't know enough about mongoDB to solve the issue:

[
  {
    '$match': {
      'prosthetistID': new ObjectId('6126ca6148f34c00189f86f5')
    }
  }, {
    '$lookup': {
      'from': 'users', 
      'localField': 'user', 
      'foreignField': '_id', 
      'as': 'userInfo'
    }
  }, {
    '$unwind': {
      'path': '$userInfo'
    }
  }
]

After the $unwind, the resulting object has a userInfo key, that contains an amputationInfo array nested:

varman · Accepted Answer · 2021-08-26 03:34:02Z

1

You can have following stages

$unwind to deconstruct the array
first $group to get the sum of each category
second $group to push into one document and make it as key value pair
$arrayToObject to get the desired output
$replaceRoot to make the data output into root

Here is the code

db.collection.aggregate([
  { "$unwind": "$userInfo.amputationInfo" },
  {
    "$group": {
      "_id": "$userInfo.amputationInfo",
      "count": { "$sum": 1 }
    }
  },
  {
    $group: {
      _id: null,
      data: { $push: {
          k: "$_id",
          v: "$count"
        }
      }
    }
  },
  { $project: { data: { "$arrayToObject": "$data" } } },
  { "$replaceRoot": { "newRoot": "$data" } }
])

Working Mongo playground

answered Aug 26, 2021 at 3:34

varman

8,9226 gold badges25 silver badges56 bronze badges

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

2 Comments

klaurtar1 Over a year ago

Ok that is exactly as I was looking for. To take this further...what if I wanted the output to be an array like [{Single Above-Elbow: 2, Double Symes/Boyd: 2}] to sort by the largest value?

varman Over a year ago

@klaurtar1 But these are key value pair where keys are unique. So I dont think (Im not sure) there is a method to sort inside object, but you can sort inside array like mongoplayground.net/p/gIrVtfQmHAC

Collectives™ on Stack Overflow

Looping through array to count in mongodb/mongoose

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