ActiveRecord的选择，通过多发HABTM

Question

我有三个型号：

class User < ActiveRecord::Base 
    has_and_belongs_to_many :groups 
end 

class Group < ActiveRecord::Base 
    has_and_belongs_to_many :channels 
    has_and_belongs_to_many :users 
end 

class Channel < ActiveRecord::Base 
    has_and_belongs_to_many :groups 
end 

什么是最有效的方式来获得特定用户的所有通道（不重复）？

那么有效：

SELECT DISTINCT name FROM users 
    JOIN groups_users ON users.id=groups_users.user_id 
    JOIN channels_groups ON groups_users.group_id=channels_groups.group_id 
    JOIN channels ON channels_groups.channel_id=channels.id; 

Answer 1

这应做到：

user = User.first 
channels = user.groups.flat_map(&:channels).uniq