MongoDB: upserting array element

Question

I'm trying to upsert a specific array element in a MongoDB document.

this is an example of a document from my collection:

{
  "_id":3,
  "name":alex,
  "array": [
    {
      "_id":1,
      "a":30,
      "b":60
    },
    {
      "_id":2,
      "a":40,
      "b":80
    }
  ]
}

In case where "array._id" exist I want to update its "a" value without changing "b" value. Output example:

{
  "_id":3,
  "name":alex,
  "array": [
    {
      "_id":1,
      "a":90,
      "b":60
    },
    {
      "_id":2,
      "a":40,
      "b":80
    }
  ]
}

In case where "array._id" doesn't exist I want to create such entry with the given "_id" and "a" values in "array". Output example:

{
  "_id":3,
  "name":alex,
  "array": [
    {
      "_id":1,
      "a":30,
      "b":60
    },
    {
      "_id":2,
      "a":40,
      "b":80
    },
    {
      "_id":4,
      "a":50,
    }
  ]
}

In case where "array" doesn't exist I want to create it and insert to it an entry with the given "_id" and "a" values. Output example:

{
  "_id":3,
  "name":alex,
  "array": [
    {
      "_id":1,
      "a":30,
    }
  ]
}

nimrod serok · Accepted Answer · 2023-08-15 15:24:45Z

One option is to use update with pipeline:

Use $reduce to iterate over the array and change the item if it is there (using $mergeObjects). Use $ifNull to handle the case where the array do not exists.
Use $concatArrays with $cond to add the item if it does not exist after the first stage.

const obj = {"_id": 1, "a": 90};
db.collection.update(
  {"_id":3},
  [{$set: {array: {
        $reduce: {
          input: {$ifNull: ["$array", [obj]]},
          initialValue: [],
          in: {$concatArrays: [
              "$$value",
              {$cond: [
                  {$eq: ["$$this._id", obj._id]},
                  [{$mergeObjects: ["$$this", obj]}],
                  ["$$this"]
              ]}
          ]}
        }
  }}},
  {$set: {
      array: {$concatArrays: [
          "$array",
          {$cond: [
              {$in: [obj._id, "$array._id"]},
              [],
              [obj]
          ]}
      ]}
  }}]
)

See how it works on the playground example

If the order of the items in the array is not important you can simplify it to one stage using $filter:

Add the new item merged with the old one if exists, to an array where the item is filtered out.

const obj = {"_id": 1, "a": 90};
db.collection.update(
  {"_id":3},
  [{$set: {array: {
        $concatArrays: [
          {$filter: {
              input: {$ifNull: ["$array", []]},
              cond: {$ne: ["$$this._id", obj._id]}
          }},
          [{$mergeObjects: [
                {$first: {$filter: {
                      input: "$array",
                      cond: {$eq: ["$$this._id", obj._id]}
                }}},
                obj
          ]}]
        ]
  }}]
)

See how it works on the playground example

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