Mongoose: what's up with "_doc"?

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.

_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.