get_products.rb

category.posts
#=> [] # list of all posts

posts.categories
#=> [] # list of all categories

scopes.rb

class Post
  has_and_belongs_to_many :categories

  scope :in_categories, lambda do |cat_ids|
    joins(:categories)
      .where(id: cat_ids)
      .uniq
  end
end

structure.rb

class Category
  has_and_belongs_to_many :posts
end

class Post
  has_and_belongs_to_many :categories
end