Can I update a document base on collection condition?-CodePudding

I would Like to make sure some condition is met before performing an update operation.

Is this possible with MongoDB in a single query?

For example, I have the following collection A:

{"_id":"1", "content":{"a":1, "b":2}},
{"_id":"2", "content":{"a":1, "b":2}},

I would like to perform an update on a document where "_id" is "1", only if there is no document that has "_id":"2" in this entire collection A.

CodePudding user response：

You can use a $lookup to check for the existence of doc _id:"2". After that, based on the $lookup result to perform $merge to update back to doc _id:"1".

db.collection.aggregate([
  {
    $match: {
      _id: "1"
    }
  },
  {
    "$lookup": {
      "from": "collection",
      "let": {
        docId: "2"
      },
      "pipeline": [
        {
          $match: {
            $expr: {
              $eq: [
                "$_id",
                "$$docId"
              ]
            }
          }
        }
      ],
      "as": "doc2"
    }
  },
  {
    $match: {
      doc2: []
    }
  },
  {
    $addFields: {
      someNewFields: "..."
    }
  },
  {
    $project: {
      doc2: false
    }
  },
  {
    "$merge": {
      "into": "collection",
      "on": "_id",
      "whenMatched": "replace"
    }
  }
])

Here is the Mongo playground that doc _id:"2" exists.

Here is the Mongo playground that doc _id:"2" does not exist.