Fixes in belongsTo relation definition syntax

lib/model.js

@@ -512,6 +512,12 @@ AbstractClass.include = function (objects, include, cb) {
         }
         var relation = relations[relationName];
 
+        if (!relation) {
+            return function() {
+                cb(new Error('Relation "' + relationName + '" is not defined for ' + self.modelName + ' model'));
+            }
+        }
+
         var req = {'where': {}};
 
         if (!keyVals[relation.keyFrom]) {
@@ -957,6 +963,9 @@ AbstractClass.belongsTo = function (anotherClass, params) {
     params = params || {};
     if ('string' === typeof anotherClass) {
         params.as = anotherClass;
+        if (params.model) {
+            anotherClass = params.model;
+        } else {
             var anotherClassName = anotherClass.toLowerCase();
             for(var name in this.schema.models) {
                 if (name.toLowerCase() === anotherClassName) {
@@ -964,12 +973,13 @@ AbstractClass.belongsTo = function (anotherClass, params) {
                 }
             }
         }
+    }
     var methodName = params.as || i8n.camelize(anotherClass.modelName, true);
     var fk = params.foreignKey || methodName + 'Id';
 
     this.relations[params['as']] = {
         type: 'belongsTo',
-        keyFrom: params['foreignKey'],
+        keyFrom: fk,
         keyTo: 'id',
         modelTo: anotherClass,
         multiple: false