Added the chronological navigation to the post page.

While following Nick's post series I felt the need for easy navigation from
current post to previous and next post in the series, thus avoiding going
back to the index page again and again.

Also it is helpful for somebody coming in from a search engine to read the
posts in order, or for archive pages.

This branch can be safely merged with the delete_handler branch and is kept
separate just as a choice to the user. The delete takes care of adjusting the
previous and next links automatically.

generators.py

@@ -73,6 +73,28 @@ def get_resource_list(cls, post):
   def get_etag(cls, post):
     return post.hash
 
+  @classmethod
+  def get_prev_next(cls, post):
+    """Retrieves the chronologically previous and next post for this post""" 
+    import models
+
+    q = models.BlogPost.all().order('-published')
+    q.filter('published <', post.published)
+    prev = q.fetch(1)
+
+    q = models.BlogPost.all().order('published')
+    q.filter('published >', post.published)
+    next = q.fetch(1)
+    if len(prev) > 0:
+      prev = prev[0]
+    else:
+      prev = None
+    if len(next) > 0:
+      next = next[0]
+    else:
+      next = None
+    return prev,next
+
   @classmethod
   def generate_resource(cls, post, resource):
     import models
@@ -83,10 +105,41 @@ def generate_resource(cls, post, resource):
     template_vals = {
         'post': post,
     }
+    prev, next = cls.get_prev_next(post)
+    if prev is not None:
+      template_vals['prev']=prev
+    if next is not None:
+      template_vals['next']=next
     rendered = utils.render_template("post.html", template_vals)
     static.set(post.path, rendered, config.html_mime_type)
 generator_list.append(PostContentGenerator)
 
+class PostPrevNextContentGenerator(PostContentGenerator):
+  """ContentGenerator for the blog posts chronologically before and after the blog post."""
+
+  @classmethod
+  def get_resource_list(cls, post):
+    prev, next = cls.get_prev_next(post)
+    resource_list = [res.key().id() for res in (prev,next) if res is not None]
+    return resource_list 
+
+  @classmethod
+  def generate_resource(cls, post, resource):
+    import models
+    post = models.BlogPost.get_by_id(resource)
+    if post is None:
+      return
+    template_vals = {
+        'post': post,
+    }
+    prev, next = cls.get_prev_next(post)
+    if prev is not None:
+     template_vals['prev']=prev
+    if next is not None:
+     template_vals['next']=next
+    rendered = utils.render_template("post.html", template_vals)
+    static.set(post.path, rendered, config.html_mime_type)
+generator_list.append(PostPrevNextContentGenerator)
 
 class ListingContentGenerator(ContentGenerator):
   path = None

models.py

@@ -58,6 +58,7 @@ def summary_hash(self):
     return hashlib.sha1(str(val)).hexdigest()
 
   def publish(self):
+    regenerate = False
     if not self.path:
       num = 0
       content = None
@@ -67,9 +68,12 @@ def publish(self):
         num += 1
       self.path = path
       self.put()
+      # Force regenerate on new publish. Also helps with generation of
+      # chronologically previous and next page.
+      regenerate = True 
     if not self.deps:
       self.deps = {}
-    for generator_class, deps in self.get_deps():
+    for generator_class, deps in self.get_deps(regenerate=regenerate):
       for dep in deps:
         if generator_class.can_defer:
           deferred.defer(generator_class.generate_resource, None, dep)

themes/default/post.html

@@ -15,6 +15,12 @@ <h2>{{post.title|escape}}</h2>
   <p class="postmeta">
     <span class="date">{{post.published|date:"d F, Y"}}</span>
   </p>
+  {% if prev %}
+    <a id="prev" href="{{prev.path}}">Previous Post</a>
+  {% endif %}
+  {% if next %}
+    <a id="next" href="{{next.path}}">Next Post</a>
+  {% endif %}
 
   {% if config.disqus_forum %}
     <h3 id="comments">Comments</h3>