Does Mongoose Actually Validate the Existence of An Object Id?

I'm using mongoose-id-validator. Works good

var mongoose = require('mongoose');
var idValidator = require('mongoose-id-validator');

var ReferencedModel = new mongoose.Schema({name: String});

var MySchema = new mongoose.Schema({
  referencedObj : { type: mongoose.Schema.Types.ObjectId, ref: 'ReferencedModel'},
  referencedObjArray: [{ type: mongoose.Schema.Types.ObjectId, ref: 'ReferencedModel' }]
});

MySchema.plugin(idValidator);

I'm doing it with middleware, performing a search of the element on validation:

ExampleSchema = new mongoose.Schema({

    parentId: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'Example'
    }

});

ExampleModel = mongoose.model('Example', ExampleSchema);

ExampleSchema.path('parentId').validate(function (value, respond) {

    ExampleModel.findOne({_id: value}, function (err, doc) {
        if (err || !doc) {
            respond(false);
        } else {
            respond(true);
        }
    });

}, 'Example non existent');

No, an ObjectId field that's defined in your schema as a reference to another collection is not checked as existing in the referenced collection on a save. You could do it in Mongoose middleware, if needed.