Model类增加readonly方法

library/think/Model.php

@@ -1018,6 +1018,21 @@ abstract class Model implements \JsonSerializable, \ArrayAccess
         return $this;
     }
 
+    /**
+     * 设置只读字段
+     * @access public
+     * @param mixed $field 只读字段
+     * @return $this
+     */
+    public function readonly($field)
+    {
+        if (is_string($field)) {
+            $field = explode(',', $field);
+        }
+        $this->readonly = $field;
+        return $this;
+    }
+
     /**
      * 是否为更新数据
      * @access public