Rails 4 many to many association not working

Question

Ruby on rails newbie here. Trying to create a starter blog app and having trouble with many to many association between my models.

I have 2 models - Post, Category t

Answer 1

When using the has_and_belongs_to_many association you need a unique index on your join table. Your migration should look like this:

class CreateCategoriesPosts < ActiveRecord::Migration
  def change
    create_table :categories_posts do |t|
      t.integer :category_id
      t.integer :post_id
      t.timestamps
    end
    add_index :categories_posts, [:category_id, :post_id]
  end
end

You can also get rid of the CategoriesPost model, that is only needed if you wanted to implement a :has_many, :through association. That should answer your question.

---

And just to be thorough, if you wanted to use a :has_many, :through association with a CategoriesPost model you can implement that like so:

class Post < ActiveRecord::Base
  has_many :categoriesposts
  has_many :categories, :through => :categoriesposts
end
class Category < ActiveRecord::Base
  has_many :categoriesposts
  has_many :posts, :through => :categoriesposts
end
class CategoriesPost < ActiveRecord::Base
  belongs_to :post
  belongs_to :category
end

Implementing this method allows you to add more attributes to your categoriespost model if you wanted.

Answer 2

Further to the first answer, you need to put associations in your join model (CategoriesPosts) like this:

Class CategoriesPosts  < ActiveRecord::Base
    belongs_to :category
    belongs_to :post
End