如何进行多级公会？

Question

我有这样的设置：

Continent -> 国家 -> 城市 -> Post

和我

class Continent < ActiveRecord::Base
   has_many :countries
end

class Country < ActiveRecord::Base
  belongs_to :continent
  has_many :cities
end

class City < ActiveRecord::Base
  belongs_to :country
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :city
end

如何通过此关联获得所有有帖子的大洲，

例如：

@posts = Post.all

@posts.continents #=> [{:id=>1,:name=>"America"},{...}] 

Answer 1

你可以这样做：

Continent.all(:joins => {:countries => {:cities => :posts}}).uniq

或这样：

class Continent < ActiveRecord::Base
  has_many :countries

  named_scope :with_post, :joins => {:countries => {:cities => :posts}}
end

# And then
Continent.with_post.uniq

或这样：

Post.all(:include => {:city => {:country => :continent}}).map { |post| post.city.country.continent }.uniq

或这样：

class Post < ActiveRecord::Base
  belongs_to :city

  named_scope :include_continent, :include => {:city => {:country => :continent}}

  def continent
    city.try(:country).try(:continent)
  end
end

# And then
Post.include_continent.map(&:continent).uniq