code the search post by tags.
 Update the doc.
bundle update.
migration to remove the post_id from tags' table.
create_posts_tags_join migration
set the has_and_belongs_to_many relations in the models.
run migration.
migration cleanup.
If no post for a tag...
a way to deals with 'has and belongs to many' in rails 3... relations throught a join table.
model cleanup.
gen docs