Challenge 2: has_many :through for Tags — Solution

# app/models/post.rb
class Post < ApplicationRecord
  has_many :taggings
  has_many :tags, through: :taggings
end

# app/models/tag.rb
class Tag < ApplicationRecord
  has_many :taggings
  has_many :posts, through: :taggings
end

# app/models/tagging.rb
class Tagging < ApplicationRecord
  belongs_to :post
  belongs_to :tag
end

# Getting all tags for a given post:
@post.tags

=begin
Notes:
- Tagging is the join model -- it's the only one of the three with two
  belongs_to declarations, one for each side of the many-to-many
  relationship it connects.
- Post and Tag are structurally symmetric: each has_many :taggings
  (its direct, one-to-many relationship to the join table) plus
  has_many :X, through: :taggings (the actual many-to-many relationship
  reached by going through that join table).
- @post.tags silently performs a posts -> taggings -> tags join behind
  one method call, the exact three-table SQL JOIN the chapter contrasted
  this against having to write by hand.
- A Tagging table would need its own migration (following the Chapter 5
  pattern) with post_id and tag_id foreign key columns for any of this to
  actually work against a real database.
=end