Polymorphism 101

post.rb

class Post < ActiveRecord::Base
  has_many :tag_links
  has_many :tags, :through => :tag_links

  # ...
end

project.rb

class Project < ActiveRecord::Base
  has_many :tag_links
  has_many :tags, :through => :tag_links

  # ...
end

tag.rb

# Schema info:
# 
# t.string  :name

class Tag < ActiveRecord::Base
  has_many :tag_links
end

tag_link.rb

# Schema info:
#
# t.integer :tag_id
# t.integer :taggable_id
# t.string  :taggable_type

class TagLink < ActiveRecord::Base
  belongs_to :tag
  belongs_to :taggable, :polymorphic => true
end