Is there a way to perform a "dry run" of an update operation?

Question

I am in the process of changing the schema for one of my MongoDB collections. (I had been storing dates as strings, and now my application stores them as ISODates; I need to go back and change all of the old records to use ISODates as well.) I think I know how to do this using an update, but since this operation will affect tens of thousands of records I'm hesitant to issue an operation that I'm not 100% sure will work. Is there any way to do a "dry run" of an update that will show me, for a small number of records, the original record and how it would be changed?

Edit: I ended up using the approach of adding a new field to each record, and then (after verifying that the data was right) renaming that field to match the original. It looked like this:

db.events.find({timestamp: {$type: 2}})
    .forEach( function (e) {
        e.newTimestamp = new ISODate(e.timestamp);
        db.events.save(e);
    } )

db.events.update({},
    {$rename: {'newTimestamp': 'timestamp'}},
    {multi: true})

By the way, that method for converting the string times to ISODates was what ended up working. (I got the idea from this SO answer.)

My advice would be to add the ISODate as a new field. Once confirmed that all looks good you could then unset the the string date. — James Wahlin, May 17 '13 at 13:48
@JamesWahlin Has the right idea, even in SQL which supports dry runs I would not do this — Sammaye, May 17 '13 at 14:04

score 7 · Accepted Answer · answered May 17 '13 at 17:23

7

My advice would be to add the ISODate as a new field. Once confirmed that all looks good you could then unset the the string date.

answered May 17 '13 at 17:23

James Wahlin

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score 6 · Answer 2 · answered May 17 '13 at 13:51

6

Create a test environment with your database structure. Copy a handful of records to it. Problem solved. Not the solution you were looking for, I'm sure. But, I believe, this is the exact circumstances that a 'test environment' should be used for.

answered May 17 '13 at 13:51

eidsonator

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I thought of that. But if my first attempt turns out not to work, I'd have to clear the development environment and re-import the data--potentially many times--and that seemed like a hassle that I might be able to avoid. – bdesham May 17 '13 at 13:56
Yeah, I assumed that had been considered, just thought I'd throw the suggestion out there, anyway... I occasionally overlook the simple stuff sometimes, myself. – eidsonator May 17 '13 at 13:58

score 2 · Answer 3 · answered May 17 '13 at 13:52

2

Select ID of particular records that you would like to monitor. place in the update {_id:{$in:[<your monitored id>]}}

answered May 17 '13 at 13:52

Dewfy

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That's one approach, although it still opens the possibility of destroying data if I screw up the update operation. – bdesham May 17 '13 at 14:55
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@bdesham it is bad practice to experiment with live data without backup. It is thumb-finger-rule for any production manipulations - create rollback point first – Dewfy May 17 '13 at 16:53

score 0 · Answer 4 · answered Aug 14 '17 at 14:14

Another option which depends of the amount of overhead it will cause you - You can consider writing a script, that performs the find operation, add printouts or run in debug while the save operation is commented out. Once you've gained confidence you can apply the save operation.

 var changesLog = [];
var errorsLog = [];
events.find({timestamp: {$type: 2}}, function (err, events) {
    if (err) {
        debugger;
        throw err;
    } else {
        for (var i = 0; i < events.length; i++) {
            console.log('events' + i +"/"+(candidates.length-1));
            var currentEvent = events[i];
            var shouldUpdateCandidateData = false;

            currentEvent.timestamp = new ISODate(currentEvent.timestamp);


            var change = currentEvent._id;
            changesLog.push(change);

            // // ** Dry Run **
            //     currentEvent.save(function (err) {
            //         if (err) {
            //             debugger;
            //             errorsLog.push(currentEvent._id + ", " + currentEvent.timeStamp + ', ' + err);
            //             throw err;
            //         }
            //     });
        }
        console.log('Done');
        console.log('Changes:');
        console.log(changesLog);
        console.log('Errors:');
        console.log(errorsLog);
        return;
    }
});

Is there a way to perform a "dry run" of an update operation?

4 Answers4