example.rb

example.rb

# post.rb
class Post
  has_many :tags, :through => :post_tags
  has_many :post_tags
end

# tag.rb
class Tag
  has_many :posts, :through => :post_tags
  has_many :post_tags
end

# post_tag.rb
# This is your join table
class PostTag
  belongs_to :tag
  belongs_to :post
end

# Then in your code when you need to set something:

tag = Tag.find_by name: "important"
post = Post.find_by title: "Foo bar"

post.tags << tag

# This is your join relationship between the post and the tag
post_tag = post.post_tags.find_by tag_id: tag.id

post_tag.some_custom_field = "the thing you want to save"
post_tag.save