The correct way of storing document reference in one-to-one relationship in MongoDB

Question

I have two MongoDB collections user and customer which are in one-to-one relationship. I'm new to MongoDB and I'm trying to insert documents manually although I have Mongoose installed. I'm not sure which is the correct way of storing document reference in MongoDB.

I'm using normalized data model and here is my Mongoose schema snapshot for customer:

/** Parent user object */
user: {
    type: Schema.Types.ObjectId,
    ref: "User",
    required: true
}

user

{ 
    "_id" : ObjectId("547d5c1b1e42bd0423a75781"), 
    "name" : "john", 
    "email" : "[email protected]", 
    "phone" : "01022223333", 
}

I want to make a reference to this user document from the customer document. Which of the following is correct - (A) or (B)?

customer (A)

{ 
    "_id" : ObjectId("547d916a660729dd531f145d"), 
    "birthday" : "1983-06-28", 
    "zipcode" : "12345", 
    "address" : "1, Main Street", 
    "user" : ObjectId("547d5c1b1e42bd0423a75781")
}

customer (B)

{ 
    "_id" : ObjectId("547d916a660729dd531f145d"), 
    "birthday" : "1983-06-28", 
    "zipcode" : "12345", 
    "address" : "1, Main Street", 
    "user" : {
        "_id" : ObjectId("547d5c1b1e42bd0423a75781")
    }
}

Answer 1

Use variant A. As long as you don't want to denormalize any other data (like the user's name), there's no need to create a child object.

This also avoids unexpected complexities with the index, because indexing an object might not behave like you expect.

Even if you were to embed an object, _id would be a weird name - _id is only a reserved name for a first-class database document.