validations: support multi-key unique constraint

Modify the "unique" validator to accept additional property names to
narrow the space of rows searched for duplicates.

Example:

Consider `SiteUser` belongsTo `Site` via `siteId` foreign key.
Inside every site, the user email must be unique. It is allowed to
register the same email with multiple sites.

    SiteUser.validateUniquenessOf('email', { scopedTo: ['siteId'] });

lib/validations.js

@@ -197,9 +197,19 @@ Validatable.validateAsync = getConfigurator('custom', {async: true});
  *  - Oracle
  *  - MongoDB
  *
+ * ```
+ * // The login must be unique across all User instances.
+ * User.validatesUniquenessOf('login');
+ *
+ * // Assuming SiteUser.belongsTo(Site)
+ * // The login must be unique within each Site.
+ * SiteUser.validateUniquenessOf('login', { scopedTo: ['siteId'] });
+ * ```
+
  * @param {String} propertyName  Property name to validate.
  * @options {Object} Options 
  * @property {RegExp} with Regular expression to validate format.
+ * @property {Array.<String>} scopedTo List of properties defining the scope.
  * @property {String} message Optional error message if property is not valid.  Default error message: "is not unique".
  */
 Validatable.validatesUniquenessOf = getConfigurator('uniqueness', {async: true});
@@ -297,6 +307,15 @@ function validateCustom(attr, conf, err, done) {
 function validateUniqueness(attr, conf, err, done) {
   var cond = {where: {}};
   cond.where[attr] = this[attr];
+
+  if (conf && conf.scopedTo) {
+    conf.scopedTo.forEach(function(k) {
+      var val = this[k];
+      if (val !== undefined)
+        cond.where[k] = this[k];
+    }, this);
+  }
+
   this.constructor.find(cond, function (error, found) {
     if (error) {
       return err();

test/validations.test.js

@@ -1,5 +1,6 @@
 // This test written in mocha+should.js
 var should = require('./init.js');
+var async = require('async');
 
 var j = require('../'), db, User;
 var ValidationError = j.ValidationError;
@@ -172,6 +173,41 @@ describe('validations', function () {
       })).should.not.be.ok;
     });
 
+    it('should support multi-key constraint', function(done) {
+      var EMAIL = 'user@xample.com';
+      var SiteUser = db.define('SiteUser', {
+        siteId: String,
+        email: String
+      });
+      SiteUser.validatesUniquenessOf('email', { scopedTo: ['siteId'] });
+      async.waterfall([
+        function automigrate(next) {
+          db.automigrate(next);
+        },
+        function createSite1User(next) {
+          SiteUser.create(
+            { siteId: 1, email: EMAIL },
+            next);
+        },
+        function createSite2User(user1, next) {
+          SiteUser.create(
+            { siteId: 2, email: EMAIL },
+            next);
+        },
+        function validateDuplicateUser(user2, next) {
+          var user3 = new SiteUser({ siteId: 1, email: EMAIL });
+          user3.isValid(function(valid) {
+            valid.should.be.false;
+            next();
+          });
+        }
+      ], function(err) {
+        if (err && err.name == 'ValidationError') {
+          console.error('ValidationError:', err.details.messages);
+        }
+        done(err);
+      });
+    });
   });
 
   describe('format', function () {