ActiveRecord: Uninitialze constant exception when define many-to-many relationship

Question

I have a model named Post and a model named Category. Relationship between Post and Category is many-to-many. So I create a join table as following:

create_table :categories_posts do |t|
  t.integer :post_id, index: true
  t.integer :category_id, index: true

  t.timestamps
end

Here is my model Post (file name: post.rb)

class Post < ApplicationRecord
  has_many :categories, :through => :categories_posts
  has_many :categories_posts
end

Here is my model CategoryPost (file name: category_post.rb)

class CategoryPost < ApplicationRecord
  self.table_name = "categories_posts"
  belongs_to :post
  belongs_to :category
end

But when I try: Post.last.categories or Post.last.categories_posts I meet exception:

NameError: uninitialized constant Post::CategoriesPost

Please tell me where I am wrong.

Thanks

Answer 1

NameError: uninitialized constant Post::CategoriesPost

The plural form of CategoryPost is CategoryPosts, so you should use category_posts instead of categories_posts when defining associations

class Post < ApplicationRecord
  has_many :category_posts
  has_many :categories, :through => :category_posts
end

However, if you want to use categories_posts, you can do with by defining class_name on the association

class Post < ApplicationRecord
  has_many :categories_posts, class_name: "CategoryPost"
  has_many :categories, :through => :categories_posts
end