Do I need to manually create a migration for a HABTM join table?

Question

I'm struggling now to get HATBM working correctly. I have a beaten scanario: articles and tags. I presume, HABTM should be used here, since it is a many-to-many relationship. I don't know however if I should manually create a join table (articles_tags in this case).

My code currently as follows:

class Article < ActiveRecord::Base
  has_and_belongs_to_many :tags  
end

class Tag < ActiveRecord::Base
  has_and_belongs_to_many :articles
end

When I run the migrations, no 3rd table is created. Also, I would like to add that my third table doesn't bear any domain logic, just blind assignment.

I'm using Rails 2.2.2

Extension

Even at rails 4, I'm guessing the answer to this is .... "yes"? :( – Hominy

@dtc, still yes, still true. – Extension

Answer 1

You should do this in a migration of one of the tables, or in a separate migration if those migrations have been ran:

create_table :articles_tags, :id => false do |t|
  t.references :article, :tag
end

add_index :articles_tags, [:article_id, :tag_id]

This will create the table for you and the :id => false tells Rails not to add an id field to this table. There's an index also, which will speed up lookups for this join table.

You could also generate a model (ArticlesTag) for this and do:

# article.rb
has_many :articles_tags
has_many :tags, :through => :articles_tags

# tag.rb
has_many :articles_tags
has_many :articles, :through => :articles_tags

# article_tag.rb
belongs_to :tag
belongs_to :article

And then create the table in the migration generated from the script/generate model articles_tag call.

Illustrator

Funny that official guides or documentation does not state this. I had to dig on Stackoverflow. Ty. – Neurology

What if you wanted to relate them on with a unique name for the join because that join was already being used. – Paske

I think you should add indexes to join table by adding add_index :articles_tags, [:article_id, :tag_id] before the end of change method – Semeiology

Should it not be :articles_tag (singular) in the model? – Mauricio

I've added create_table :articles_tags... to my Tag migration. What I should do so my rake db:rollback works correctly? – Baluster

Answer 2

Note that this is covered in the API.

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many

Dirkdirks

Though it ought to be mentioned in a guide someplace. – Ruthieruthless

Read these few lines in the API so you understand the expected name of the table (e.g. when using the same prefix). This will explain the comment of @Giovanni . – Ulcerate

Answer 3

You probably also want to add an index to the migration:

add_index "articles_tags", "article_id"

add_index "articles_tags", "tag_id"

However, if you want tagging functionality I'd recommend the acts_as_taggable_on rails plugin:

http://www.intridea.com/tag/acts_as_taggable_on http://github.com/mbleigh/acts-as-taggable-on/

I've used it on a project and it was very easy to implement.

One of the issues with a join table for tagging is that it can easily get ugly creating a join table for each content type you wish to make taggable (ie. comments_tags, posts_tags, images_tags, etc). This plugin uses a taggings table which includes a discriminator to determine the content type without the need of a specific join table for each type.

Ruscher

Migration methods references (or belongs_to) automatically adds indexes. You do not need that. You can break your migration with it. – Neurology

Migration methods references (or belongs_to) really do not automatically adds indexes. (Thought they do :-) – Neurology

IMHO it would be better to create a composite primary key. But it's database specific. – Gautious

Answer 4

In combination with this Qeuestion(1st answear) How to set up a typical users HABTM roles relationship and 1st answear from here, it has to be understood even by a monkey. I am new in RoR and it's got working like a charm

Revisionist