Musings about FP and CS

A log of my journey through FP and CS

TypeSafe URLs in Lift

by Clément Delafargue on August 3, 2011

Tagged as: archives.

This article is part of a series of articles I have extracted from my previous blog. Most of these articles are out of date, and some of them are terribly misguided.

I’ve been working for a few months with Lift, a really nice web framework. Lift takes ideas and concepts from a few other frameworks. The main difference between Lift and other frameworks is that Lift does not enforce MVC (even though you can do MVC if you want to). Lift’s approach is view-first, which means you first create the view, then you tell which part of the code will take care of dynamism.

For more propaganda: Seven Things and Web Framework Manifesto

I’ll explain how to build REST-y URLs in a type-safe manner, with the SiteMap. The goal is to access a blog post with this kind of URL: /posts/{id}

I’ll illustrate this article with something tremendously original: a Blog, with posts. I’ll use mongoDB as a backend, but it’s not really important.

The model

It’s a simple blog post. Let’s just assume the author is stored in plain text.

class BlogPost private() extends MongoRecord with ObjectIdPk[BlogPost]
{
 def meta = BlogPost
 object author extends StringField(this, 32)
 object date extends DateField(this)
 object content extends TextareaField(this, 500)

 val url = ... // We'll see that later
}

object BlogPost extends BlogPost with MongoMetaRecord[BlogPost] {
 val entry = ... // We'll see that later
 def calcHref(bp: BlogPost) = ... // We'll see that later
}

The template

Nothing special there. Just save it in webapp/posts.html. We have a h1 for the title, a p for some info, and a p for the actual post.

<div class='lift:surround?with=default;at=content'>
<div class='lift:BlogPostSnippet'>
<h1 class='title'></h1>
<p class='info'></p>
<p class='blogpost'></p>
</div>
</div>

The snippet

This class will be instantiated with the blog post we want to display. We just inject data in the markup.

class BlogPostSnippet(val blogPost: BlogPost) {
 def render = '.title *' \#> blogPost.title.is &
 '.info *' \#> ('By ' + blogPost.author.is) &
 '.blogpost *' \#> blogPost.content.is
}

The SiteMap

So, back to BlogPost.entry, where the magic happens. We define an entry of the sitemap which is associated with the BlogPost class. The two first arguments are the id and the displayed name of the link. The third argument is a function which takes a String and returns a Box[BlogPost]. This tells Lift how to instantiate BlogPostSnippet with the right blog post. The fourth argument is used to create an url from a given blog post. Finally, we specify the address and tell Lift we don’t want this entry to show up in the menus.

The calcHref method is just a shortcut.

class BlogPost ... {
 /* Snip */
 val url = BlogPost.calcHref(this)
}

object BlogPost ... {
 val entry = Menu.param[BlogPost](
 'blogpost',
  S ? 'blogpost.view',
 (id: String) => BlogPost.find(id),
 (bp: BlogPost) => bp.id.is
 ) / 'posts' / * >> Hidden

 def calcHref(bp: BlogPost) = entry.toLoc.calcHref(bp)
}

Don’t forget to add BlogPost.entry to the SiteMap

val entries = /* other entries */ :: BlogPost.entry :: Nil
LiftRules.setSiteMap(SiteMap(entries: _*))

Next time I’ll show you how to display comments after the blog post, while keeping the HTML structure of every comment in the template :)

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