var userSchema = new Schema({
     username: { type: String, trim: true, unique: true },
     password: { type: String, trim: true },
     firstname: { type: String, trim: true },
     lastname: { type: String, trim: true }
  });

'use strict'
    var db = require('../usermodel.js');

    exports.up = function(db, next) {
        db.rpush('username', 'admin');
        next();
   };

db.rpush is not a function