cat.js

// Example:
//   var catModel = require('./cat');
//   var Cat = catModel();
//   var cat = new Cat({ name: 'mike' });
//   cat.say(); //=> myao! i'm mike!
//
//   var mongoose = require('mongoose');
//   catModel({ mongoose: new mongoose.Mongoose() });

var defineModel = function(mongoose, modelName, defineSchema) {
  var modelArgs = [modelName];
  if (mongoose.modelNames().indexOf(modelName) < 0) {
    modelArgs.push(defineSchema(mongoose));
  }
  return mongoose.model.apply(mongoose, modelArgs);
};

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

  Schema.methods.say = function() {
    return 'myao! i\'m ' + this.name + '!';
  };

  return Schema;
};

module.exports = function(options) {
  var mongoose = options.mongoose ? options.mongoose : require('mongoose');
  if (options.mongoose) delete options.mongoose;
  return defineModel(mongoose, 'Cat', defineSchema);
};