Use standard NodeJS callbacks to validate MongooseJS documents.

document-validation.js

/**
 * Josh Wright <@BendyTree>
 * http://www.joshwright.com
 *
 * MIT
 *
 *
 * Standard MongooseJS validation looks like this:
 *
 *     schema.path("email").validate(function(val, callback){
 *       var isValid = /^.+@.+[.].+$/.test(val);
 *       callback(isValid);
 *     }, "The email address is invalid.");
 *
 * It leaves a bit to be desired:
 *
 *   - The callback expects a falsy value if there's an error (the opposite of standard callbacks).
 *   - The validation must be attached to a path (sometimes validation includes multiple).
 *   - You cannot customize the error message based on the error.
 *
 * To use the following validation, you need to extend mongoose like so:
 *
 *     var mongoose = require('mongoose');
 *     require('./document-validation')(mongoose);
 *
 * Then you can use standard node callbacks:
 *
 *     var schema = new mongoose.Schema({...});
 *     schema.addValidator(function(callback){
 *       callback("Your email address must contain an @ sign.");
 *     });
 *
 */

module.exports = function(mongoose) {

  mongoose.Schema.prototype.addValidator = function (validator) {
    var schema = this;
    var pathKeys = _.keys(schema.paths);

    //make sure there is a path we can attach to
    if (!pathKeys.length)
      throw new Error("Unable to add validator to schema because schema has no keys.");

    //add validator using normal path validation
    schema.path(pathKeys[0]).validate(function(val, isValidCallback){
      //keep track of the doc
      var doc = this;

      //run the custom validator then adapt the standard callback to Mongoose weirdness
      validator.call(doc, function(err){
        //if there was an error, call `invalidate` so the error message was maintained
        if (err) {
          doc.invalidate("", err);
        }

        //always report that original validation succeeded since it
        //doesn't allow us to customize the error message
        isValidCallback(true);
      });
    });
  };

};