This example is making use of the CouchDB Python Library by Christopher Lenz and is derived from his example blog application.

You´ll have to create a database named tagging_example and a design document named _design/posts with the CouchDB Views below.

CouchDB Views

all_tags

map 

function(doc) {
  if (doc.type == 'post' && doc.tags) {
    doc.tags.forEach(function(tag) {
      emit(tag, 1);
    });
  }
}

reduce 

function(keys, values) {
  return sum(values);
}

by_tag

map 

function(doc) {
  if (doc.type == 'post' && doc.tags) {
    doc.tags.forEach(function(tag) {
      emit(tag.slug, doc);
    });
  }
}

Create a Post model

Tags are entered as comma separated strings in a TextField called tagstring and will be generated from it on storing a post. As this is just a simple example, splitting the tagstring and making slugs in a sensible way is up to you. 

   1 from couchdb.client import Server
   2 from couchdb.schema import Document, Schema, DictField, ListField, TextField
   3 
   4 server = Server('http://127.0.0.1:5984/')
   5 db = server['tagging_example']
   6 
   7 
   8 class Post(Document):
   9 
  10     type = TextField(default='post')
  11 
  12     title = TextField(default='')
  13     # ... rest of your post model: slug, body, created, etc.
  14 
  15     tagstring = TextField(default='')
  16     tags = ListField(DictField(Schema.build(
  17                     title = TextField(),
  18                     slug  = TextField()
  19                     )))
  20 
  21     def store(self, db):
  22         self.tags = self._make_tags()
  23         Document.store(self, db)
  24 
  25     @classmethod
  26     def all_tags(cls):
  27         return make_cloud([(row.key, row.value) for row in
  28                 db.view('_view/posts/all_tags', group=True)])
  29 
  30     @classmethod
  31     def by_tag(cls, **options):
  32         return cls.view(db, '_view/posts/by_tag', **options)
  33 
  34     def _make_tags(self):
  35         taglist = set([tag.strip() for tag in self.tagstring.split(',')])
  36         tags = [
  37                 dict(
  38                     title = tag,
  39                     slug  = tag.lower()
  40                     ) for tag in taglist
  41                 ]
  42         return tags
  43 
  44 
  45 def make_cloud(tags):
  46     """
  47     Calculation taken from Zine blogging engine.
  48     """
  49     import math
  50     cloud = []
  51     for tag in tags:
  52         tag[0][u'size'] = 100 + math.log(tag[1] or 1) * 20
  53         cloud.append(tag[0])
  54     return cloud

Add some posts with tagstrings:

   1 >>> p1 = Post(title=u"My first Post", tagstring=u"CouchDB, tagging, example")
   2 >>> p1.store(db)
   3 >>> p2 = Post(title=u"My second Post", tagstring=u"CouchDB, RESTful, HTTP, JSON, API")
   4 >>> p2.store(db)
   5 >>> p3 = Post(title=u"My third Post", tagstring=u"CouchDB, Erlang, HTTP")
   6 >>> p3.store(db)

Output

Looking at the all_tags design document in Futon you´ll see this table:

key

value

{"slug": "api", "title": "API"}

1

{"slug": "couchdb", "title": "CouchDB"}

3

{"slug": "erlang", "title": "Erlang"}

1

{"slug": "example", "title": "example"}

1

{"slug": "http", "title": "HTTP"}

2

{"slug": "json", "title": "JSON"}

1

{"slug": "restful", "title": "RESTful"}

1

{"slug": "tagging", "title": "tagging"}

1

The method of the Post model that calls the all_tags design document returns: 

   1 >>> list(Post.all_tags())
   2 [{u'size': 100.0, u'slug': u'api', u'title': u'API'},
   3  {u'size': 121.9722457733622, u'slug': u'couchdb', u'title': u'CouchDB'},
   4  {u'size': 100.0, u'slug': u'erlang', u'title': u'Erlang'},
   5  {u'size': 100.0, u'slug': u'example', u'title': u'example'},
   6  {u'size': 113.8629436111989, u'slug': u'http', u'title': u'HTTP'},
   7  {u'size': 100.0, u'slug': u'json', u'title': u'JSON'},
   8  {u'size': 100.0, u'slug': u'restful', u'title': u'RESTful'},
   9  {u'size': 100.0, u'slug': u'tagging', u'title': u'tagging'}]

Getting posts by a specific tag: 

   1 >>> list(Post.by_tag()["example"])
   2 [<Post u'7d2894971759039454ef29be57cc1b81'@u'1141652034' {u'tagstring': u'CouchDB, tagging, example', u'tags': [{u'slug': u'couchdb', u'title': u'CouchDB'}, {u'slug': u'example', u'title': u'example'}, {u'slug': u'tagging', u'title': u'tagging'}], u'type': u'post', u'title': u'My first Post'}>]


   1 >>> list(Post.by_tag()["couchdb"])
   2 [<Post u'13b107ad3e14e06352b2476e015cf72f'@u'686077196' {u'tagstring': u'CouchDB, RESTful, HTTP, JSON, API', u'tags': [{u'slug': u'api', u'title': u'API'}, {u'slug': u'http', u'title': u'HTTP'}, {u'slug': u'json', u'title': u'JSON'}, {u'slug': u'couchdb', u'title': u'CouchDB'}, {u'slug': u'restful', u'title': u'RESTful'}], u'type': u'post', u'title': u'My second Post'}>,
   3  <Post u'7d2894971759039454ef29be57cc1b81'@u'1141652034' {u'tagstring': u'CouchDB, tagging, example', u'tags': [{u'slug': u'couchdb', u'title': u'CouchDB'}, {u'slug': u'example', u'title': u'example'}, {u'slug': u'tagging', u'title': u'tagging'}], u'type': u'post', u'title': u'My first Post'}>,
   4  <Post u'd71d7d8233a318745124b82f1f8a99d2'@u'667119109' {u'tagstring': u'CouchDB, Erlang, HTTP', u'tags': [{u'slug': u'http', u'title': u'HTTP'}, {u'slug': u'couchdb', u'title': u'CouchDB'}, {u'slug': u'erlang', u'title': u'Erlang'}], u'type': u'post', u'title': u'My third Post'}>]

ToDo