Mongoose: what's up with "_doc"?

Question

It seems Mongoose is doing something really funky internally.

1 var Foo = new mongoose.model('Foo', new mongoose.Schema({a: String, b: Number}));
2 var foo = new Foo({a: 'test; b: 42}); 
3 var obj = {c: 1};
4 foo.goo = obj;                  // simple object assignment. obj should be 
                                  //   passed by reference to foo.goo. recall goo
                                  //   is not defined in the Foo model schema

5 console.log(foo.goo === obj);   // comparison directly after the assignment
    // => false, doesn't behave like normal JS object

Essentially, any time you try to deal with properties of a Mongoose model that aren't

a) defined in the model's schema or

b) defined as the same type (array, obj, ..) ... the model doesn't even behave like a normal Javascript object.

Switching line 4 to foo._doc.goo = obj makes the console output true.

edit: trying to reproduce weirdness

example 1:

 // Customer has a property 'name', but no property 'text'
 // I do this because I need to transform my data slightly before sending it
 // to client.
 models.Customer.find({}, function(err, data) {
     for (var i=0, len=data.length; i<len; ++i) {
        data[i] = data[i]._doc;            // if I don't do this, returned data
                                           // has no 'text' property
        data[i].text = data[i].name;       
    }
    res.json({success: err, response:data});
});

Answer 1

Update

Maybe I misunderstood your original question, but now it looks like the nature of your question changed, so the below information isn't relevant, but I'm leaving it. :)

I tested your code and it works fine for me. Mongoose doesn't execute any special code when you set properties that aren't part of the schema (or a few other special properties). JavaScript currently doesn't support calling code for properties that don't yet exist (so Mongoose can't get in the way of the set of the goo property for example).

So, when you set the property:

foo.goo = { c: 1 };

Mongoose isn't involved. If your console.log was something other than the code you displayed, I could see that it might report incorrectly.

Additionally, when you send the results back as JSON, JSON.stringify is being called, which calls toString on your Mongoose Model. When that happens, Mongoose only uses the properties defined on the schema. So, no additional properties are being sent back by default. You've changed the nature of the data array though to directly point at the Mongoose data, so it avoids that problem.

Details about normal behavior

When you set the property goo using Mongoose, quite a few things happen. Mongoose creates property getters/setters via the Object.defineProperty (some docs). So, when you set the goo property, which you've defined as a [String], a few things happen:

1. Mongoose code is called prior to the value being set onto the object instance (unlike a simple JavaScript object)
2. Mongoose creates an array (optionally) to store the data (a MongooseArray) which will contain the array data. In the example you provided, since you didn't pass an array, it will be created.
3. Mongoose will attempt to cast your data to the right type
4. It will call toString on the data passed as part of the cast.

So, the results are that the document now contains an array with a toString version of the object you passed.

If you checked the contents of the goo property, you'd see that it's now an array with a single element, which is a string that contains [object Object]. If you'd picked a more basic type or matched the destination property storage type, you would see that a basic equality check would have worked.

Answer 2

_doc exist on the mongoose object.

Because mongooseModel.findOne returns the model itself, the model has structure (protected fields). When you try to print the object with console.log it gives you only the data from the database, because console.log will print the object public fields.

If you try something like JSON.stringify then you get to see inside the mongoose model object. (_doc, state ...)

In the case where you want to add more fields in the object and it's not working

const car = model.findOne({_id:'1'})
car.someNewProp = true // this will not work

If later you set the property to the object car and you didn't specify in the Model Schema before then Mongoose model is validating if this field exists and if it's the valid type. If the validation fails then the property will not be set.

Answer 3

I was stuck on this today... Drove me nuts. Not sure if the below is a good solution (and OP has mentioned it too), but this is how I overcame this issue.

My car object:

 cars = [{"make" : "Toyota"}, {"make" : "Kia"}];

Action:

console.log("1. Cars before the color: " + car);

cars.forEach(function(car){
     car.colour = "Black";   //color is NOT defined in the model. 
});

console.log("2. Cars after the color: " + car);

Problematic console output:

   1. Cars before the color: [{"make" : "Toyota"}, {"make" : "Kia"}];
   2. Cars after the color: [{"make" : "Toyota"}, {"make" : "Kia"}];   //No change! No new colour properties :(

If you try to pass in this property that was undefined in the model, via doc (e.g. car._doc.color = "black"), it will work (this colour property will be assigned to each car), but you can't seem to access it via EJS (frontend) for some reason.

Solution: (Again, not sure if this is the best way... but it worked for me): Add in this new property (colour) in the car model.

var carSchema = mongoose.Schema({
   make: String,
   color: String   //New property.
})

With the model redefined, everything worked as normal / expected (no _doc 'hacks' needed etc.) and I lived another day; hope it helps someone else.

Answer 4

Had same problem. Instead of updating my model.

const car = model.findOne({_id:'1'})
let temp = JSON.stringify(car);
let objCar = JSON.parse(temp);
objCar.color = 'Red'; //now add any property you want

this solves my problem

Answer 5

There is some weirdness with Mongoose models and you have to check that Mongoose doesn't already have a model created in it's models array.

Here is my solution:

import mongoose from 'mongoose';
createModel = (modelName="foo", schemaDef, schemaOptions = {})=> {
  const { Schema } = mongoose;
  const schema = Schema(schemaDef, schemaOptions);
  const Model = mongoose.models[modelName] || mongoose.model(modelName, schema);
  return Model;
}

I use my own mongoose model class and base class for my models. I made this and it should work for you.

Answer 6

Try using lean

By default, Mongoose queries return an instance of the Mongoose Document class. Documents are much heavier than vanilla JavaScript objects, because they have a lot of internal state for change tracking. Enabling the lean option tells Mongoose to skip instantiating a full Mongoose document and just give you the POJO.

https://mongoosejs.com/docs/tutorials/lean.html

Answer 7

you can use toJSON() instead of _doc