# app/models/user.rb
class User < ActiveRecord::Base
  has_many :permissions

  # you can't set up a typical has_many relationship when dealing 
  # with polymorphic relationships, you must use source and source type 
  # to fully specify what type of objects you intend to retrieve

  has_many :posts, through: :permissions, source: :permitable, source_type: 'Post' do 
    # you can add any number of methods further scoping the has_many query.
    # it is advised to use Arel, however as of this writing Bitwise operations
    # are not supported in Arel so, sadly we use a string.

    def owner_of
      where('`permissions`.`bitmask` & ? > 0', Permission::OWNER )
    end
  end

  # comments looks just like posts, you could add in some meta programming to make this 
  # automatice whenever you include a module in your source class. Perhaps we'll explore
  # that later.

  has_many :comments, through: :permissions, source: :permitable, source_type: 'Comment' do
    def owner_of
      where('`permissions`.`bitmask` & ? > 0', Permission::OWNER )
    end
  end

end

# app/models/post.rb
class Post < ActiveRecord::Base
  # you have to use the as: option here to get the relationship to function
  # as expected

  has_many :permissions, as: :permitable
  has_many :users, through: :permissions
end

# app/models/comment.rb
# same as post for all intents and purposes
class Comment < ActiveRecord::Base
  has_many :permissions, as: :permitable
  has_many :users, through: :permissions
end

# app/models/permission.rb

# our permissions model holds all the logic for storing and defining permissions.
# in our case we are using a simple bitmask so the individual numbers are stored here as well
class Permission < ActiveRecord::Base
 
  # you have to use polymorphic true to make permissions use the 
  # permitable_id and permitable_type columns for the relationship
 
  belongs_to :permitable, polymorphic: true
  belongs_to :user
  
  # we are using bitwise calcs to determine permissions, you can use 
  # booleans in columns if it works better for your application
  ADMIN = 1
  OWNER = 2
  VIEW = 4
  EDIT = 8

end

# db/shema.rb

# obviously if you are using ActiveRecord you need a database schema backing all this up
# this is the most basic of schema's required to get this working.
ActiveRecord::Schema.define(version: 20160615181741) do

  create_table "comments", force: :cascade do |t|
    t.string   "email"
    t.text     "body"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "permissions", force: :cascade do |t|
    t.integer  "permitable_id"
    t.string   "permitable_type"
    t.integer  "user_id"
    t.integer  "bitmask",         default: 0
    t.datetime "created_at",                  null: false
    t.datetime "updated_at",                  null: false
  end

  add_index "permissions", ["bitmask"], name: "index_permissions_on_bitmask"
  add_index "permissions", ["permitable_type", "permitable_id"], name: "index_permissions_on_permitable_type_and_permitable_id"
  add_index "permissions", ["user_id"], name: "index_permissions_on_user_id"

  create_table "posts", force: :cascade do |t|
    t.string   "title"
    t.text     "body"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "users", force: :cascade do |t|
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

end