mongoose-schema - are both children array + parent doc's ID necessary? - node.js

Question

I'm using a couple of one-to-many models and was wondering what the advantage of having both an array of "children" ObjectID()s and a "parent" model's ObjectID() in the child is. For example:
// a client will have a new card every ten visits
var ClientSchema = new Schema({
first_name: String,
last_name: String,
email: String,
cards: [] // ObjectID()s, <--- is this necessary?
});
var CardSchema = new Schema({
client: ObjectID(), // <--- or is this enough?
visits: []
});
I think the client: ObjectID() should do the trick in most cases, specially with the Population options Mongoose offers.

Answer 1

It suffices to store the reference ObjectId in one of the documents.
As you can read in the documentation or in this answer, the reference field needs a type and a ref. The name of field is arbitrary. Once you have done this and registered your models, you can use the models to populate your queries.
var clientSchema = new Schema({
first_name: String,
last_name: String,
email: String
});
var cardSchema = new Schema({
client: {
type: Schema.Types.ObjectId,
ref: 'client'
}
});
// models
var Card = mongoose.model('Card', cardSchema);
var Client = mongoose.model('Client', clientSchema);
Yet, it could be helpful to store an array of ObjectId's. However, this also creates a huge window for mistakes. A mismatch of foreign key and primary key, in SQL dialect. You can always do a count query if you need to count. I would say, do either the above or do this:
var clientSchema = new Schema({
first_name: String,
last_name: String,
email: String,
cards: [
{
type: Schema.Types.ObjectId,
ref: 'card'
}
]
});
var cardSchema = new Schema({
card_name: String
});
// models
var Card = mongoose.model('Card', cardSchema);
var Client = mongoose.model('Client', clientSchema);