Sails-beforeBlueprint-Policy

beforeBlueprint.js

/**
 * beforeBlueprint
 *
 * @module      :: Policy
 * @description :: Simple policy to enable hooks into the model which can act upon req, res objects.
 * @docs        :: http://sailsjs.org/#!documentation/policies
 *
 */

var actionUtil = require( 'sails/lib/hooks/blueprints/actionUtil' );

capitaliseFirstLetter = function ( string ) {
  return string.charAt( 0 ).toUpperCase() + string.slice( 1 );
};

module.exports = function ( req, res, next ) {

  var Model = actionUtil.parseModel( req );
  var blueprint = capitaliseFirstLetter( req.options.action ); // we actually enable this for all actions, not only blueprints

  if ( typeof Model[ "controlBefore" + blueprint ] === "function" ) {
    var result = Model[ "controlBefore" + blueprint ]( req, res, next );
    //next();
  } else {
    sails.log.debug( 'Policy beforeBlueprint: Model "' + Model.identity + '" has no method controlBefore' + blueprint + '.' );
    next();
  }

};

user.js

// Model defintion api/models/user.js

module.exports = {

  attributes: {
    name: "string",
    "posts": {
      collection: "post",
      via: "user"
    }
  },

  // beforeFind hook to modify the request before it hits the find blueprint
  // other hooks possible, with the naming schema of controlBefore*Blueprint
  controlBeforeFind: function ( req, res, next ) {
    // modify req.options.where OR req.body OR req.query as you need
    next();
  }
};

@IanVS
This is a workaround to implement custom lifecycle callback. To check unique username before user creation you can use the default lifecycle callback beforeCreate in your model like:

beforeCreate: function(values, cb) {
   ...
 },

Please refer docs for details.