Umbraco create new document type in razor

@using umbraco.cms.businesslogic.web

@* From https://gist.github.com/joeriks/877600
  A Razor script to add a basic comment function to Umbraco pages 
 You need a document type called Comment with a textstring property 
 called commentName and a textbox multiple property called commentMessage 
 You also need to allow the Comment document type as child document type 
 to your textpage document type. 
 Create this script with a macro and add it below the bodyText in your 
 template(s) *@

@* Some global variables *@

@{
  var successfulPost = false;
  var name = Request["name"];
  var message = Request["message"];
}

@* Handle form post: create a new document if a valid post*@

@if (IsPost)
{
  if (name!="" && message !="")
  {
    var dt = DocumentType.GetByAlias("Comment");
 
    @* Make sure its a valid document type alias *@
   
    if (dt != null)
    {
   
      var author = umbraco.BusinessLogic.User.GetUser(0);

      var doc = Document.MakeNew("Comment", dt, author, Model.Id);
      doc.getProperty("commentName").Value = name;
      doc.getProperty("commentMessage").Value = message;
 
      @* Tell umbraco to publish the document *@
     
      doc.Publish(author);
      umbraco.library.UpdateDocumentCache(doc.Id);
     
      successfulPost = true;
    }
  }
}

@* Render the Html *@

<hr/>

@if (successfulPost)
{
 <h3>Successful post</h3>
 <p>Thank you <strong>@name</strong> for your message:</p>
 <p>@message</p>
} 

<h3>Comments:</h3>

@foreach (dynamic c in Model.Comment)
{
  <div>
    <p>@c.commentMessage</p>
    <p>By : @c.commentName (at @c.CreateDate)</p>
  </div>
}

<h3>Add a new comment:</h3>
  
<form method="post">
  <fieldset>
    <label for="name">Your name</label>
    <input type="text" name="name" id="name"/><br/>
    <label for="message">Comment</label>
    <textarea name="message" id="message"></textarea><br/>
    <input type="submit" value="Post comment"/>
  </fieldset>
</form>