ruby-on-rails - 根据嵌套模型的计数进行排序的 Rails 查询？

Question

我有两个模型:Group & GroupMember

我想返回按最高 GroupMember 计数排序的组？你如何在 Rails 中设置这种类型的连接/查询计数？

Answer 1

有了 counter_cache，生活就轻松多了:

$ rails g migration add_group_member_counts_to_groups

# migration
def change
  add_column :groups, :group_members_count, :integer, default: 0
end

class GroupMember < ActiveRecord::Base
  belongs_to :group, counter_cache: true
end

class Group < ActiveRecord::Base
  has_many :group_members
  scope :order_by_group_members_count, order('group_members_count DESC')
end

Group.order_by_group_members_count # => What you're looking for

只有一个查询，只在组表上。

编辑

如果您需要重置 group_members_count:

ids = Group.select(:id).map &:id
ids.each do |id|
  Group.reset_counters(id, :group_members)
end

(因为 reset_counters 不接受 id 数组)

以后 Rails 3.2.0 ，你会写:

ids = Group.pluck(:id)

我等不及了 ;-)