如何多层次关联？

Question

我有这样的设置：

Continent - >Country - >City - >Post

，我有

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 

如何获得各大洲有帖子低谷这个协会

Like：

@posts = Post.all 

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

Answer 1

你可以这样做：

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

或者这样：

class Continent < ActiveRecord::Base 
    has_many :countries 

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

# And then 
Continent.with_post.uniq 

或者这样：

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

或者这样：

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