在 has_many 上按计数排序：通过

Question

我使用 Rails 4。我有一个应用程序，我有一个多对多关系：

class User < ActiveRecord::Base
  has_many :relationshipfollows, :foreign_key => "follower_id",
                           :dependent => :destroy
  has_many :following, :through => :relationshipfollows, :source => :followed
end

class Project < ActiveRecord::Base
 has_many :relationshipfollows, :foreign_key => "followed_id",
                           :dependent => :destroy
 has_many :followers, :through => :relationshipfollows, :source => :follower
end

class Relationshipfollow < ActiveRecord::Base
  belongs_to :follower, :class_name => "User"
  belongs_to :followed, :class_name => "Project"
end

我遵循这个教程：http://ruby.railstutorial.org/chapters/following-users?version=3.0#top

但现在我想列出所有按关注者数量排序的项目。像这样：

1. project1 | 99 followers
2. project2 | 16 followers
3. project3 | 2 followers
...

我是 Rails 新手，我想我一直在犯错误，因为我尝试了很多这样的例子： Rails 3 Order By Count on has_many :through 或has_many , through: relationship count

我试试这个方法： Project.joins(:relationshipfollows).group("relationshipfollows.project_id").order("count(relationshipfollows.project_id) desc")

但我有这个错误：SQLite3::SQLException: no such column: relationshipfollows.project_id: SELECT "projects".* FROM "projects" INNER JOIN "relationshipfollows" ON "relationshipfollows"."followed_id" = "projects"."id" GROUP BY relationshipfollows.project_id ORDER BY count(relationshipfollows.project_id) desc

我尝试了另一种方法：

Project.joins(:relationshipfollow).select('following.*, COUNT(followers.id) AS user_count').group('project_id').order('COUNT(followers.id) DESC')

但我有这个错误：Association named 'relationshipfollow' was not found on Project; perhaps you misspelled it?

有人能帮我找到正确的方向吗？

真诚的

编辑： 我认为问题出在此处。当我尝试这个时：

Relationshipfollow.select(:followed_id, "COUNT(follower_id) AS total").group(:followed_id).order("total DESC")

他还给我这个：

=> #ActiveRecord::Relation [#Relationshipfollow id:nil,followed_id:2,#Relationshipfollow id:nil,followed_id:1, # 关系follow id：nil，followed_id：3]

所有项目都按关注者数量排序，并且所有followed_id（项目）相对于我的测试都处于良好的顺序。但是当我像这样将它加入我的模型项目时：

Project.joins(:relationshipfollows).select(:followed_id, "COUNT(follower_id) AS total").group(:followed_id).order("total DESC")

他返回给我一个项目列表，但 project_id 为 NULL：

=> #ActiveRecord::Relation [# Project id: nil, # Project id: nil, # Project id: nil]

Answer 1

我会这样做：

#app/models/project.rb
Class Project < ActiveRecord::Base
     has_many :relationshipfollows, :foreign_key => "follower_id", :dependent => :destroy
     has_many :followers, :through => :relationshipfollows, :source => :follower

     scope :sort_by_followers, -> { joins(:followers).select("followers.*", "COUNT(followers.id) AS follower_count").group(:project_id).order("follower_count DESC") }
end

#app/controllers/projects_controller.rb
def index
    @projects = Project.sort_by_followers
end

#app/views/projects/index.html.erb
<ol>
    <% @projects.each_with_index do |project, i| %>
        <li><%= "Project#{i} | #{project.follower_count} followers" %></li>
    <% end %>
</ol>

Answer 2

Project.joins(:relationshipfollows) 将进行内部连接。因此，没有关注者的项目将不会包含在结果集中。你需要这样的东西：

Project.select("projects.*, COUNT(relationshipfollows.follower_id) as follower_count").joins("LEFT OUTER JOIN relationshipfollows ON relationshipfollows.followed_id = projects.id").group("projects.id").order("follower_count DESC")

Answer 3

我的救援解决方案是在我的项目模型中添加一个整数 numberfollowers

但是有了另一个项目和 rails 指南 (http://guides.rubyonrails.org/active_record_querying.html)，我终于找到了我的要求的答案

Project.joins(:relationshipfollows).select('projects.*, COUNT(followed_id) as user_count').group('projects.id').order('user_count DESC')