What is the "__v" field in Mongoose

346

I'm using Mongoose version 3 with MongoDB version 2.2. I've noticed a __v field has started appearing in my MongoDB documents. Is it something to do with versioning? How is it used?

This question is tagged with node.js mongodb mongoose

~ Asked on 2012-09-19 13:35:18

The Best Answer is


341

From here:

The versionKey is a property set on each document when first created by Mongoose. This keys value contains the internal revision of the document. The name of this document property is configurable. The default is __v.

If this conflicts with your application you can configure as such:

new Schema({..}, { versionKey: '_somethingElse' })

~ Answered on 2012-09-19 13:37:29


83

Well, I can't see Tony's solution...so I have to handle it myself...


If you don't need version_key, you can just:

var UserSchema = new mongoose.Schema({
    nickname: String,
    reg_time: {type: Date, default: Date.now}
}, {
    versionKey: false // You should be aware of the outcome after set to false
});

Setting the versionKey to false means the document is no longer versioned.

This is problematic if the document contains an array of subdocuments. One of the subdocuments could be deleted, reducing the size of the array. Later on, another operation could access the subdocument in the array at it's original position.

Since the array is now smaller, it may accidentally access the wrong subdocument in the array.

The versionKey solves this by associating the document with the a versionKey, used by mongoose internally to make sure it accesses the right collection version.

More information can be found at: http://aaronheckmann.blogspot.com/2012/06/mongoose-v3-part-1-versioning.html

~ Answered on 2015-08-07 07:47:32


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