Replace HABTM by HMT with Join Table as Model

habtm_hmt.md

Has And Belongs To Many (use case)

how it works ?

Let's take the simple example of a blog with:

• A post habtm tags
• A tag habtm posts

how is it represented in rails ?

Let's assume that a Post and Tag model already exist. This is the migration for the join table

class CreatePostsTags < ActiveRecord::Migration[5.1]
  def change
    create_table :posts_tags, id: false do |t|
      t.belongs_to :post, index: true
      t.belongs_to :tag, index: true
    end
  end
end

And the relations in each model

class Post < ApplicationRecord
  has_and_belongs_to_many :tags
end

class Tag < ApplicationRecord
  has_and_belongs_to_many :posts
end

Cool! it works !

Few months later, let's assume that you want to order tags in each post for searching purposes.

HABTM to has_many :through

why ?

In our case, has_many :through permits to declare the join table as model. And so, in order to operate (filter, sort, allow, reject, etc..) at a join table level. That permits to store the rank of the tag in the post's tag list direclty in the join model.

Migration issue

class CreatePostsTags < ActiveRecord::Migration[5.1]
  def change
    create_table :posts_tags, id: false do |t|
      t.belongs_to :post, index: true
      t.belongs_to :tag, index: true
    end
  end
end

A model needs a primary key to be retrieved. By convention rails uses the id column as primary key. The problem is that we've removed the id column from our join table (id: false). this makes sense because we just need the post_id and tag_id to retrieve a record.

But in order to use has_many :through, we need to add a primary key to the posts_tags table. So how to do so ?

class AddPrimaryKeyAndRankToPostsTags < ActiveRecord::Migration[5.1]
  def change
    rename_table 'posts_tags', 'post_tags'
    add_column :post_tags, :id, :primary_key
    add_column :post_tags, :rank, :integer, default: 0
  end
end

Then in models

class PostTag < ApplicationRecord
  belongs_to :post
  belongs_to :tag
end

class Post < ApplicationRecord
  has_many :post_tags, -> { order(rank: :asc) }
  has_many :tags, through: :post_tags
end

class Tag < ApplicationRecord
  has_many :post_tags
  has_many :posts, through: :post_tags
end

Voilà !

habtm_schema.png

this helped me better understand the shift from HABTM to HMT, thank you!

Thank you for the kind words!

Hope that helps!!