Why does Mongoose add blank arrays?

Question

I am trying to start using Mongoose as an ODM for MongoDB with my node.js application. I have noticed that when I design a schema with an embedded document that if I don't add a value to it, it store a blank array "[]" in Mongo. Why is this? I am trying to store historical changes to records and a blank array would mean that that change deleted the value. Here is a sample schema.

schema.Client = new mongoose.Schema({
    name:{type:String, required:true},
    products:[{
        name:{type:String, index:true},
        startDate:Date,
        endDate:Date
    }],
    subdomain:{type:String, index:{unique:true}},
})

Here is the resulting document when I save a document with just name and subdomain.

{
    "name": "Smith Company",
    "products": [],
    "subdomain": "smith"
}

Why did it add products with a blank array by default and how can I stop it?

the default value for an array is an empty array, so if you save only {name:"foo",subdomain:"bar"} products will be an empty array. — supernova, Sep 30 '12 at 05:02
Is there any way to change the default value for an array to result in it not writing anything? — wintzer, Sep 30 '12 at 05:07
i'm still on mongoose 2.x , you may want to have a look at http://mongoosejs.com/docs/api.html#model_Model-update , try setting safe to false. — supernova, Sep 30 '12 at 05:24
Giving an empty array special meaning over a non-existent array is tempting, but it's a bad idea because of issues like this. It's better to add an explicit boolean flag to make the distinction you require. — JohnnyHK, Sep 30 '12 at 14:29
Possible duplicate of [mongoose remove empty objects or arrays](http://stackoverflow.com/questions/32979302/mongoose-remove-empty-objects-or-arrays) — maxko87, Oct 16 '15 at 18:50
Odd... I want mongoose to create empty arrays in the schema but it's not doing it. Any ideas? — dmr07, Mar 14 '16 at 06:59

score 40 · Answer 1 · answered Sep 11 '17 at 13:55

40

You can workaround by define the schema like below:

products: {
  type: [{
    name:String,
    startDate:Date,
    endDate:Date
  }],
  default: undefined
}

answered Sep 11 '17 at 13:55

Angelo Chen

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score 23 · Answer 2 · answered Sep 30 '12 at 15:17

23

Blank array gives you convenient way to add or remove elements from your Model.

$push $addToSet $pull in update would help you to manage your array elements.

If you don't have a blank array then you cannot push elements to null

But it is possible in blank array.

answered Sep 30 '12 at 15:17

jwchang

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There are still cases though where you would want the lack of an attribute to stand for "irrelevant for this document". When that is a common thing, it's an ugly waste to have a bunch of empty arrays. It seems to defeat one of the beauties of document databases, namely the ability to store different document structures in the same collection. I would like to see the ability for blank arrays to result in no attribute in the document unless a blank array is explicitly part of the model being saved. This is also intuitive. That or an explanation for why it's not possible in MongoDB. – neverfox Jul 12 '13 at 05:33
1

Currently in mongodb $push and $addToSet will create an array if there isn't one there. – Dobes Vandermeer Oct 14 '19 at 19:49

score 13 · Answer 3 · answered Mar 23 '19 at 07:07

13

By default the array has [] value. you can overwrite that :

var UserSchema = new Schema({
  followers : {
    type: [String],
    default: undefined
  }
})

answered Mar 23 '19 at 07:07

Muslim Omar

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jasoncrawford · Answer 4 · 2013-09-03T20:16:37.427

3

This seems to be by design, but there is a workaround here using a 'pre' handler to remove the default empty array: https://github.com/LearnBoost/mongoose/issues/1335

This only worked for me when I set the field to null, though. If I set it to undefined as in the sample code, the empty array seems to come back.

edited Sep 03 '13 at 20:16

answered Sep 03 '13 at 19:55

jasoncrawford

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score 0 · Answer 5 · answered Sep 30 '12 at 02:35

0

Because in your schema you are defining products to be an array of objects. You would need to try something like:

products: {
        name:{type:String, index:true},
        startDate:Date,
        endDate:Date
    },

This will store an empty object instead of an array.

answered Sep 30 '12 at 02:35

Menztrual

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I want it to be an array, as one doc may contain references to multiple products. I don't want it to write anything to the database if I don't put any data in it. – wintzer Sep 30 '12 at 05:04
1

I think this isn't even written to the database - have you checked? I had a model without an array, and when I just added an array of object ids and `.findOne`'d one of these Model instances the `.toJSON` added the empty list in the response, even though I haven't written to the database since. – s-ol Feb 04 '15 at 12:39

Why does Mongoose add blank arrays?

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