mongodb mongoose nodejs express4为什么在对象数组字段中自动插入_id？

Question

It might be conceptual question about _id in mongodb.

I understand mongodb will insert a _id field automatically if you don't set key field in document.In my case, I defined a field as Object Array, I don't know why it always create a _id in each Object in Array of this field.

I do appreciate if someone could clarify it for me.

Mongoose Model Scheme definition:

module.exports = mongoose.model("Application", {
    Name: String,
    Description: String,
    Dependency: [
            {
            App_id: {
                type: mongoose.Schema.Types.ObjectId,
                ref: 'Application'
            },
            Priority: Number
        }
    ]
});

This is an Update operation, request data is:

{ _id: '571953e33f33c919d03381b5',
  Name: 'A Test Utility (cmd)',
  Description: 'A Test Utility (cmd)'
  Dependency:
   [ { App_id: '571953e33f33c919d03381b6', Priority: true },
     { App_id: '571953e33f33c919d03383da', Priority: 0 } ] 
}

I use this code to update it

var id = req.body._id;
    Application.findOneAndUpdate({ _id: id }, req.body, function (err, app) {
        if (err)
            res.send(err);
        res.json(app);
    });    

The update is successful.But the document in mongodb is:

{
    "_id" : ObjectId("571953e33f33c919d03381b5"),
    "Name" : "A Test Utility (cmd)",
    "Description" : "A Test Utility (cmd)",
    "Dependency" : [ 
        {
            "Priority" : 1,
            "App_id" : ObjectId("571953e33f33c919d03381b6"),
            "_id" : ObjectId("571a7f552985372426509acb")
        }, 
        {
            "Priority" : 0,
            "App_id" : ObjectId("571953e33f33c919d03383da"),
            "_id" : ObjectId("571a7f552985372426509aca")
        }
    ]
}

I just don't understand how come the _id in the "Dependency" Array?

Thanks.

Answer 1

When you use [{..}] that means inside it act as a sub schema and you know that MongoDB insert a _id field automatically if you don't set key field in document. So you need to force to insert document without _id field.

Need use {_id:false} for your Dependency array schema to insert without _id

var ApplicationSchema = new mongoose.Schema({
    Name: String,
    Description: String,
    Dependency: [
        {
            App_id: {
                type: mongoose.Schema.Types.ObjectId,
                ref: 'Application'
            },
            Priority: Number,
            _id: false
        }
   ]
});

module.exports = mongoose.model("Application", ApplicationSchema);