Allow embedsOne to use auto-generated id (from connector)

If the connector has a `generateId` method - like Transient - it can be
used to set the id (when the property has been set to `generated:
true`).

lib/relation-definition.js

@@ -1803,11 +1803,25 @@ EmbedsOne.prototype.build = function (targetModelData) {
   var modelTo = this.definition.modelTo;
   var modelInstance = this.modelInstance;
   var propertyName = this.definition.keyFrom;
+  var forceId = this.definition.options.forceId;
+  var persistent = this.definition.options.persistent;
+  var connector = modelTo.dataSource.connector;
   
   targetModelData = targetModelData || {};
   
   this.definition.applyProperties(modelInstance, targetModelData);
   
+  var pk = this.definition.keyTo;
+  var pkProp = modelTo.definition.properties[pk];
+  
+  var assignId = (forceId || targetModelData[pk] === undefined);
+  assignId = assignId && !persistent && (pkProp && pkProp.generated);
+  
+  if (assignId && typeof connector.generateId === 'function') {
+      var id = connector.generateId(modelTo.modelName, targetModelData, pk);
+      targetModelData[pk] = id;
+  }
+  
   var embeddedInstance = new modelTo(targetModelData);
   modelInstance[propertyName] = embeddedInstance;
   
@@ -2241,7 +2255,7 @@ EmbedsMany.prototype.build = function(targetModelData) {
   var connector = modelTo.dataSource.connector;
   
   var pk = this.definition.keyTo;
-  var pkProp = modelTo.definition.properties[pk]
+  var pkProp = modelTo.definition.properties[pk];
   var pkType = pkProp && pkProp.type;
   
   var embeddedList = this.embeddedList();

test/relations.test.js

@@ -1877,6 +1877,37 @@ describe('relations', function () {
     
   });
   
+  describe('embedsOne - generated id', function () {
+    
+    before(function () {
+      tmp = getTransientDataSource();
+      db = getSchema();
+      Person = db.define('Person', {name: String});
+      Passport = tmp.define('Passport',
+        {id: {type:'string', id: true, generated:true}},
+        {name: {type:'string', required: true}}
+      );
+    });
+
+    it('can be declared using embedsOne method', function (done) {
+      Person.embedsOne(Passport);
+      db.automigrate(done);
+    });
+    
+    it('should create an embedded item on scope', function(done) {
+      Person.create({name: 'Fred'}, function(err, p) {
+        should.not.exist(err);
+        p.passportItem.create({name: 'Fredric'}, function(err, passport) {
+          should.not.exist(err);
+          passport.id.should.match(/^[0-9a-fA-F]{24}$/);
+          p.passport.name.should.equal('Fredric');
+          done();
+        });
+      });
+    });
+    
+  });
+  
   describe('embedsMany', function () {
     
     var address1, address2;