#By adding a join table categories_posts in your database:

class CreateCategoriesPosts < ActiveRecord::Migration[6.0]
  def change
    create_table :categories_posts do |t|
      t.references :post, foreign_key: true
      t.references :category, foreign_key: true
      t.timestamps
    end
  end
end
#And using through in your associations instructions in your models classes:

class Post < ActiveRecord::Base
  has_many :categories_posts
  has_many :categories, through: :categories_posts
end
class Category < ActiveRecord::Base
  has_many :categories_posts
  has_many :posts, through: :categories_posts
end
#And don’t forget to add the belongs_to in the new CategoriesPost model:

class CategoriesPost < ActiveRecord::Base
  belongs_to :category
  belongs_to :post
end