diff --git a/lib/model.js b/lib/model.js
index d0d6c30..b72c123 100755
--- a/lib/model.js
+++ b/lib/model.js
@@ -25,6 +25,7 @@ var Model = function(name, toshihiko, fields, options) {
     var self = this;
 
     this.primaryKeys = [];
+    this.ai = null;
 
     this.name = name;
     this.toshihiko = toshihiko;
@@ -55,6 +56,8 @@ var Model = function(name, toshihiko, fields, options) {
         self._fieldsKeyMap.name[f.name] = f;
         self._fieldsKeyMap.column[f.column] = f;
 
+        if(f.autoIncrement) self.ai = f;
+
         return f;
     });