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?

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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.)

Answer 1

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.

Answer 2

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.

Answer 3

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