Mongoose Singleton Model

MongooseSingletonModel.js

schema.statics = {
    getSingleton: function (cb) {
        this.findOne()
            .sort({ updated: -1 })
            .limit(1)
            .exec((err, model) => {
                if (err) {
                    cb(err, null);
                } else if (!model) {
                    cb(err, new this());
                } else {
                    cb(err, model);
                }
            });
    },
};

schema.pre('save', function (next) {
    this.model('HomePage').find({}, (err, docs) => {
        if (docs.length) {
            if (docs[0]._id.toString() === this._id.toString()) {
                return next();
            } else {
                return next(new Error('Document already exists'));
            }
        } else {
            next();
        }
    });
});