Discussion: Revert on update

[rootkea]

Hello all!

This is not really a bug report but a discussion post. I would like to know how are you reverting the update op.

For update operation a proper revert will restore the initial value and not just empty it. For e.g. a = 20 and now after update a = 100. To revert means a should be set to 20 only (the previous value) and not 0 or "".

I looked at the README and the example specified there is not really reverting but just overwriting the value:

module.exports = {
  up(db) {
    return db.collection('albums').updateOne({artist: 'The Beatles'}, {$set: {blacklisted: true}});
  },

  down(db) {
    return db.collection('albums').updateOne({artist: 'The Beatles'}, {$set: {blacklisted: false}});
  }
};

In down we are just setting blacklisted to false blindly. What if the original value before the update was true for some records? This is not a true revert.

So, how do you revert the update op?

One approach I can think of is:
In up, read the db first before updating and save the data read in a new .json file as a backup. Then in down, just read that newly created backup json file and update the db.

Is it okay if we create the file on-the-fly after reading the db? Is this approach correct?