What is the best way to eliminate specific elements from an array of objects in MongoDB aggregate based on a condition?

Question

What is the best way to eliminate specific elements from an array of objects in MongoDB aggregate based on a condition?

I have a database of documents called ChatRooms stored in MongoDB with the following structure:

{
  _id: ObjectId('4654'),
  messages: [
    {
      user: ObjectId('1234'),
      sentAt: ISODate('2022-03-01T00:00:00.000Z')
    },
    {
      user: ObjectId('1234'),
      sentAt: ISODate('2022-03-02T00:00:00.000Z')
    },
    {
      user: ObjectId('8888'),
      sentAt: ISODate('2022-03-03T00:00:00.000Z')
    },
  ]
}

My goal is to filter the messages array within an aggregate pipeline so that I only get one entry for each unique userId. The desired result should look like this (or something similar where the array does not contain duplicate entries for the same user id):

{
  _id: ObjectId('4654'),
  messages: [
    {
      user: ObjectId('1234'),
      sentAt: ISODate('2022-03-01T00:00:00.000Z')
    },
    {
      user: ObjectId('8888'),
      sentAt: ISODate('2022-03-03T00:00:00.000Z')
    },
  ]
}

Is there a way to achieve this? Any assistance would be greatly appreciated.

javascript mongodb express mongoose

Answer 1

Answer №1

There are numerous methods to accomplish this task, but here is an illustration of how you can do it by utilizing the $reduce operator:

db.collection.aggregate([
  {
    $addFields: {
      messages: {
        $reduce: {
          input: "$messages",
          initialValue: [],
          in: {
            $cond: [
              {
                $in: [
                  "$$this.user",
                  "$$value.user"
                ]
              },
              "$$value",
              {
                "$concatArrays": [
                  "$$value",
                  [
                    "$$this"
                  ]
                ]
              }
            ]
          }
        }
      }
    }
  }
])

Mongo Playground

Answer 2

There are numerous methods to accomplish this task, but here is an illustration of how you can do it by utilizing the $reduce operator:

db.collection.aggregate([
  {
    $addFields: {
      messages: {
        $reduce: {
          input: "$messages",
          initialValue: [],
          in: {
            $cond: [
              {
                $in: [
                  "$$this.user",
                  "$$value.user"
                ]
              },
              "$$value",
              {
                "$concatArrays": [
                  "$$value",
                  [
                    "$$this"
                  ]
                ]
              }
            ]
          }
        }
      }
    }
  }
])

Mongo Playground

What is the best way to eliminate specific elements from an array of objects in MongoDB aggregate based on a condition?

Answer №1

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