Unknown attribute when making association in a HABTM relationship

Question

I am trying to make what should be a pretty straight forward association between two models. The models are games and categories .

I first setup the two models.

create_table "games", options: "ENGINE=InnoDB DEFAULT CHARSET=utf8", force: :cascade do |t|
  t.bigint "user_id"
  t.string "title"
  t.integer "min_play_time"
  t.integer "min_players"
  t.integer "max_players"
  t.text "description"
  t.string "image"
  t.string "rules_url"
  t.string "playthrough_url"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
  t.integer "max_play_time"
  t.integer "min_age"
  t.integer "best_number_of_players"
  t.integer "recommended_min_age"
  t.integer "bgg_number"
  t.integer "year_published"
  t.index ["user_id"], name: "index_games_on_user_id"
end

create_table "categories", options: "ENGINE=InnoDB DEFAULT CHARSET=utf8", force: :cascade do |t|
  t.bigint "bgg_id"
  t.string "name"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

Then I setup my join table using rails g migration CreateJoinTableCategoryGame Category Game .

Migration file looks like this:

class CreateJoinTableCategoryGame < ActiveRecord::Migration[5.2]
  def change
    create_join_table :Categories, :Games do |t|
      t.index [:category_id, :game_id]
      t.index [:game_id, :category_id]
    end
  end
end

I migrated my database. Then I went into the console and created a test category and game and tried to associate them, this is what I got.

Game.find(1).categories << Category.find(1)
   (0.4ms)  SET  @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'),  @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483
  Game Load (0.4ms)  SELECT  `games`.* FROM `games` WHERE `games`.`id` = 1 LIMIT 1
  Category Load (0.4ms)  SELECT  `categories`.* FROM `categories` WHERE `categories`.`id` = 1 LIMIT 1
   (0.2ms)  BEGIN
   (0.3ms)  ROLLBACK
Traceback (most recent call last):
        1: from (irb):1
ActiveModel::UnknownAttributeError (unknown attribute 'game_id' for Game::HABTM_Categories.)```

Answer 1

Run rails db:rollback to reverse previous migration of CreateJoinTableCategoryGame and then change the code to this

create_join_table :categories, :games do |t|
  t.index [:category_id, :game_id]
  t.index [:game_id, :category_id]
end

now run rails db:migrate

and run Game.find(1).categories << Category.find(1) in rails console.

This is happening because table names are in lowercase and you're passing camelcase.

Also your models should look like this:

class Game < ActiveRecord::Base
  has_many :category_games
  has_many :categories, through: :category_games
end

class Cateogry < ActiveRecord::Base
  has_many :category_games
  has_many :games, through: :category_games
end

class CategoryGame < ActiveRecord::Base
  belongs_to :game
  belongs_to :category
end