Has Many Through With ActiveAdmin on Rails

board_game.rb

# app/models/board_game.rb

class BoardGame < ApplicationRecord
  has_many :board_game_categories
  has_many :categories, through: :board_game_categories
  
  ## This line was added:
  accepts_nested_attributes_for :board_game_categories, allow_destroy: true

  has_many :board_game_mechanics
  has_many :mechanics, through: :board_game_mechanics

  has_many :board_game_designers
  has_many :designers, through: :board_game_designers

  validates :name, presence: true, uniqueness: true
end

board_games.rb

# app/admin/board_games.rb

ActiveAdmin.register BoardGame do
  permit_params :name, board_game_categories_attributes: [:id, :board_game_id, :category_id, :_destroy]

  index do
    selectable_column
    column :id
    column :name
    column :rank
    column :categories do |board_game|
      board_game.categories.map { |bg| bg.name }.join(", ").html_safe
    end
    actions
  end

  show do |board_game|
    attributes_table do
      row :name
      row :rank
      row :categories do |board_game|
        board_game.categories.map { |bg| bg.name }.join(", ").html_safe
      end
    end
  end

  form do |f|
    f.semantic_errors *f.object.errors.keys

    f.inputs "Board Game" do
      f.input :name
      f.input :rank
      f.has_many :board_game_categories, allow_destroy: true do |n_f|
        n_f.input :category
      end
    end

    f.actions
  end
end