Mongoose uniqueness validator, case-sensitive or not

mongooseUniquenessValidation.js

// context
var mongoose = require('mongoose');
mongoose.connect('mongodb://...');

/**
 * Generates Mongoose uniqueness validator
 * 
 * @param string modelName
 * @param string field
 * @param boolean caseSensitive
 * 
 * @return function
 **/
function unique(modelName, field, caseSensitive) {
  return function(value, respond) {
    if(value && value.length) {
      var query = mongoose.model(modelName).where(field, new RegExp('^'+value+'$', caseSensitive ? 'i' : undefined));
      if(!this.isNew)
        query = query.where('_id').ne(this._id);
      query.count(function(err, n) {
        respond(n<1);
      });
    }
    else
      respond(false);
  };
}

// usage:
var Account = mongoose.model('Account', {
  
  // case insensitive
  email: { type: String, required: true, validate: [unique('Account', 'email'), 'unique']}
  
  // case sensitive
  csfield: { type: String, required: true, validate: [unique('Account', 'csfield', true), 'unique']}
});

// inspirations
// http://mongoosejs.com/docs/validation.html
// http://fabianosoriani.wordpress.com/2012/03/22/mongoose-validate-unique-field-insensitive/