Question

There is going to be “articles” and “tags” in my App Engine application.

And there are two techniques to implement that (thanks to Nick Johnson’s article):

# one entity just refers others
class Article(db.Model):
  tags = db.ListProperty(Tag)

# via separate "join" table
class ArticlesAndTags(db.Model):
  article = db.ReferenceProperty(Article)
  tag = db.ReferenceProperty(Tag)

Which one should I prefer according to the following tasks?

• Create tag cloud (frequently),
• Select articles by a tag (rather rarely)

Answer 1

Because of the lack of a ‘reduce’ feature in appengine’s map reduce (nor an SQL group by like query), tag clouds are tricky to implement efficiently because you need to count all tags you have manually. Which ever implementation you go with, what I would suggest for the tag cloud is to have a separate model TagCounter that keeps track of how many tags you have. Otherwise the tag query could get expensive if you have a lot of them.

class TagCounter:
   tag = db.ReferenceProperty(Tag)
   counter = db.IntegerProperty(default=0)

Every time you choose to update your tags on an article, make sure you increment and decrement from this table accordingly.

As for selecting articles by a tag, the first implementation is sufficient (the second is overly complex imo).

class Article(db.Model):
  tags = db.ListProperty(Tag)

  @staticmethod
  def select_by_tag(tag):
    return Article.all().filter("tags", tag).run()