models.rb

class Category < ApplicationRecord 
  has_many :article_categories, class_name: ArticleCategory, foreign_key: :category_id, dependent: :destroy 
  has_many :articles, through: :article_categories 
end

class Article < ApplicationRecord 
  belongs_to :author, class_name: User, foreign_key: :author_id 
  has_many :article_categories, class_name: ArticleCategory, foreign_key: :article_id, dependent: :destroy 
  has_many :categories, through: :article_categories 
  has_many :images, class_name: ArticleImage, foreign_key: :article_id, dependent: :destroy 
end

class ArticleCategory < ApplicationRecord 
  belongs_to :article, class_name: Article, foreign_key: :article_id 
  belongs_to :category, class_name: Category, foreign_key: :category_id, counter_cache: :articles_count 
end