How to set ObjectId as a data type in mongoose

Using node.js, mongodb on mongoHQ and mongoose. I'm setting a schema for Categories. I would like to use the document ObjectId as my categoryId.

var mongoose = require('mongoose');

var Schema = mongoose.Schema,
    ObjectId = Schema.ObjectId;
var Schema_Category = new Schema({
    categoryId  : ObjectId,
    title       : String,
    sortIndex   : String
});

I then run

var Category = mongoose.model('Schema_Category');
var category = new Category();
category.title = "Bicycles";
category.sortIndex = "3";

category.save(function(err) {
  if (err) { throw err; }
  console.log('saved');
  mongoose.disconnect();     
});

Notice that I don't provide a value for categoryId. I assumed mongoose will use the schema to generate it but the document has the usual "_id" and not "categoryId". What am I doing wrong?

Unlike traditional RBDMs, mongoDB doesn't allow you to define any random field as the primary key, the _id field MUST exist for all standard documents.

For this reason, it doesn't make sense to create a separate uuid field.

In mongoose, the ObjectId type is used not to create a new uuid, rather it is mostly used to reference other documents.

Here is an example:

var mongoose = require('mongoose');

var Schema = mongoose.Schema,
    ObjectId = Schema.ObjectId;
var Schema_Product = new Schema({
    categoryId  : ObjectId, // a product references a category _id with type ObjectId
    title       : String,
    price       : Number
});

As you can see, it wouldn't make much sense to populate categoryId with a ObjectId.

However, if you do want a nicely named uuid field, mongoose provides virtual properties that allow you to proxy (reference) a field.

Check it out:

var mongoose = require('mongoose');

var Schema = mongoose.Schema,
    ObjectId = Schema.ObjectId;
var Schema_Category = new Schema({
    title       : String,
    sortIndex   : String
});

Schema_Category.virtual('categoryId').get(function() {
    return this._id;
});

So now, whenever you call category.categoryId, mongoose just returns the _id instead.

You can also create a "set" method so that you can set virtual properties, check out this link for more info

I was looking for a different answer for the question title, so maybe other people will be too.

To set type as an ObjectId (so you may reference author as the author of book, for example), you may do like:

const Book = mongoose.model('Book', {
  author: {
    type: mongoose.Schema.Types.ObjectId, // here you set the author ID
                                          // from the Author colection, 
                                          // so you can reference it
    required: true
  },
  title: {
    type: String,
    required: true
  }
});