Question

I ve created 3 models:

Article: contains an article
Tag: contains tags
ArticleTag: meant for associating a many-to-one tags to article relationship. It contains a tag_id and an article_id.

The problem I m having is I m fairly new to the active record technology and I don t understand the proper way to define everything. Currently, which I think is wrong, is I have a

ArticleTag
 belongs_to :article
 belongs_to :tag

Now, from here my thought was to then add

  Article
   :has_many :tag

I m not sure if im approaching this correctly at all. Thanks for the help!

Answer 1

It depends whether you want a join model or not. A join model lets you hold extra information against the association between two other models. For example, perhaps you want to record a timestamp of when the article was tagged. That information would be recorded against the join model.

If you don t want a join model, then you could use a simple has_and_belongs_to_many association:

class Article < ActiveRecord::Base
  has_and_belongs_to_many :tags
end

class Tag < ActiveRecord::Base
  has_and_belongs_to_many :articles  
end

With a Tagging join model (which is a better name than ArticleTag), it would look like this:

class Article < ActiveRecord::Base
  has_many :taggings
  has_many :tags, :through => :taggings
end

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :articles, :through => :taggings  
end

class Tagging < ActiveRecord::Base
  belongs_to :article
  belongs_to :tag
end

A Guide to Active Record Associations

Answer 2

You should use has_many when the relationship is one-way. An article has many tags, but tags also have many articles, so that s not quite right. A better choice might be has_and_belongs_to_many: an Article has many Tags, and those Tags also reference articles. A belongs_to B means that A references B; A has_one B means that B references A.

Here s a summary of the relationships you might see:

Article
  has_and_belongs_to_many :tags   # An article has many tags, and those tags are on
                                  #  many articles.

  has_one                 :author # An article has one author.

  has_many                :images # Images only belong to one article.

  belongs_to              :blog   # This article belongs to one blog. (We don t know
                                  #  just from looking at this if a blog also has
                                  #  one article or many.)

Answer 3

Off the top of my head, Article should be:

has_many :article_tags
has_many :tags, :through => :article_tags

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