Mongodb - update replacing a pattern of an array element

Question

Given: a document as below

{
    _id: ObjectId("5eb...."),
    jobId: '40297467',
    aliases: [aX1, bX2_0, cX1X9_0]
}

When: combinations X1...X9 are not welcome in any of aliases

Then: I want to have a mongo native elegant way to replace an appropriate regex pattern with an empty string, so, as a result, I have the document as below:

{
    _id: ObjectId("5eb...."),
    jobId: '40297467',
    aliases: [a, b_0, c_0]
}

Homework done:

Here MONGO mongodb script to replace a substring from a array element there is a solution for cases where you do not need pattern and have a simple string to replace, however, it is of not much use - since aggregation replaceAll find argument "any valid expression that resolves to a string or a null" (https://docs.mongodb.com/manual/reference/operator/aggregation/replaceAll/) So, if I use pattern it just does not work.

mongo forEach loop does the job but it looks a bit ugly for me... Besides, I'm not sure what will be the price of such looping.

db.col.find({ jobId: "40297467" }).forEach(function (x) {
  const regex = /*pattern*/gi;
  array = []
  for (let alias of x.aliases) {
    array.push(alias.replace(regex, ""));
  }
  db.col.update({ _id: x._id }, { $set: { aliases: array} });
});

Any help would be much appreciated :)

prasad_ · Accepted Answer · 2021-09-23 05:44:34Z

1

This update with aggregation pipeline works with MongoDB v4.4 (uses the $function aggregate operator):

db.collection.updateMany(
{ },
[
  { 
      $set: { 
          aliases: {
              $function: {
                  body: function(arr) {
                               arr = arr.map(e => e.replace(/x\d/gi, ""));
                               return arr;
                  },
                  args: [ "$aliases" ],
                  lang: "js"
              }
          }
      }
  }
])

answered Sep 23, 2021 at 5:44

prasad_

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3 Comments

Nick Over a year ago

just to clarify - does this solution of yours have any performance benefits in comparison with find().forEach() in the question? Besides refactor of course since I suppose line arr = arr.map(e => e.replace(/x\d/gi, "")); can be used in my solution too.

prasad_ Over a year ago

The statement arr = arr.map(e => e.replace(/x\d/gi, "")); is pure JavaScript; so you can use it in your code too. The benefit of using the update with aggregation pipeline (the solution) would be that the entire code (including the JS function) runs on the MongoDB database server. You submit the update, the update happens on the server and you get the the update confirmation. Your code deals with a read (find makes a call to the database), process on the client program and then an update on the server.

Nick Over a year ago

ok, cool, thx for the explanation and your time

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