Question 117152 in Mongodb Question posted in
The official documentation can be found here.

Update the Last Item of Nested Array By Adding to Its Own Value in MongoDB

I’m trying to make a seemingly simple update to a MongoDB collection that looks like the below using Node.

Collection

{
  account_id: "ORG1",
  progress: [{week: 1, goal: 5000, raised: 2400}, {week: 2, goal:5100,  raised: 1000}]
}

The goal is to be able to

  1. Find the correct org (this works for me)
  2. Add a value to the last array entry’s "raised" value. (e.g. Initially the raised value is 1000, after my update, it will be 1000 + an incoming value).

My hacky approach would be to do a find query to get the initial value and then do an update to add my incoming value to the initial value. But I’m sure there’s a simpler way.

Thanks in advance!

Tags:

3

Answers


  1. Chosen as BEST ANSWER
    0

    I ended up finding a solution that feels a bit simpler than the solutions I've seen.

    I added a field to the db called "status":

    {
  account_id: "ORG1",
  progress: [{week: 1, goal: 5000, raised: 2400, status: historic}, {week: 2, goal:5100,  raised: 1000, status: current}]
}

    Then I ended up using the positional operator ($) and increment functions.

    const collection = client.collection(goalCollection);

 // Search Query
const query = {
  'account_id': ORG1,
  'progress.status: "current"
};

// Update with increment
const update = {
   $inc: {"progress.$.raised": ADDITIONAL_SUM}
}


// Run the update
const result = await collection.updateOne(query, update);

    Worked smoothly.


  2. 0

    One option is using an update pipeline:

    1. Split the array into the last item and all the rest.
    2. update the last item
    3. Build the array again using $concatArrays
    db.collection.update(
  {account_id: "ORG1"},
  [
    {$set: {
      lastItem: {$last: "$progress"},
      rest: {$slice: ["$progress", 0, {$subtract: [{$size: "$progress"}, 1]}]}
    }
    },
    {$set: {"lastItem.raised": {$add: ["$lastItem.raised", incomingValue]}}},
    {$set: {
      progress: {$concatArrays: ["$rest", ["$lastItem"]]},
      lastItem: "$$REMOVE",
      rest: "$$REMOVE"
    }
  }
])

    See how it works on the playground example – concatArrays

    Another option is using $reduce:

    Here we are iterating on the array, for each item checking if it is the last one, and if so updating it:

    db.collection.update(
  {account_id: "ORG1"},
  [
  {$set: {lastIndex: {$subtract: [{$size: "$progress"}, 1]}}},
  {$set: {
      lastIndex: "$$REMOVE",
      progress: {
        $reduce: {
          input: "$progress",
          initialValue: [],
          in: {
            $concatArrays: [
              "$$value",
              [
                {
                  $cond: [
                    {$eq: [{$size: "$$value"}, "$lastIndex"]},
                    {$mergeObjects: [
                       "$$this",
                        {raised: {$add: ["$$this.raised", incomingValue]}}
                      ]
                    },
                    "$$this"
                  ]
                }
              ]
            ]
          }
        }
      }
    }
  }
])

    See how it works on the playground example – reduce

    Login or Signup to reply.
  3. 0

    One way to do it is by using "$function" in the update/aggregation pipeline.

    db.collection.update({
  "account_id": "ORG1"
},
[
  {
    "$set": {
      "progress": {
        "$function": {           // your increment value goes here топ
          "body": "function(prog) {prog[prog.length - 1].raised += 75; return prog}",
          "args": ["$progress"],
          "lang": "js"
        }
      }
    }
  }
])

    Try it on mongoplayground.net.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search