hack to determine whether an error is a validation error from waterline (for Sails/Waterline 0.9.x)

isValidationError.js

var _ = require('lodash');

/**
 * `isValidationError`
 *
 * Is this a waterline validation error?
 */

function isWaterlineValidationError (err) {
  if (_.isPlainObject(err)) {
    var keys = Object.keys(err);
    if (keys.length) {
      var failedValidation = err[keys[0]];
      if (_.isArray(failedValidation) && failedValidation.length &&
        _.isPlainObject(failedValidation[0]) && failedValidation[0]['rule']
      ) {
        return true;
      }
    }
  }
  if ( _.isString(err) && err.match(/duplicate key value violates unique constraint/g) ) {
    return true;
  }

  if ( _.isString(err) && err.match(/^Bad request/ig)) {
    return true;
  }
  return false;
}

More compact solution

function(err, model) {
    // no errors
    if (!err) return true;
    // error is not a validation error
    if (!err.ValidationError) return false;
    // { field: { rule: 'Message', otherRule: 'Message' ... } ... }
    var messages = model.validationMessages,
        errors = err.ValidationError,
        output = {};

    Object.keys(errors).forEach(function(field) {
        output[field] = [];
        errors[field].forEach(function(error) {
            output[field].push(messages[field] && messages[field][error.rule] || error.rule);
        })
    });

    return output;
}

Note that in Sails v1 this is changing a bit. For details, see the docs on sailsjs.com (or balderdashy/sails#3459 (comment) for a bit more background)