总结 created_at 和 updated_at 之间的差异（Rails ActiveRecord - 寻找更好的方法）

Question

TL;DR：

所以我想我会的：

@user.resources.each do |r|
  #Something like this...
  r.views.sum(:created_at, :group=>:resource_id) - r.views.sum(:updated_at, :group=>:resource_id)
end

现在，我知道我可以让它像这样工作：

r = Resource.first
time_sum_difference = r.views.sum {|v| v.updated_at.to_i - v.created_at.to_i}

但我宁愿让 ActiveRecord 为我计算它

编辑：废话，显然不能对时间戳求和，需要就地计算差异

有没有更好的方法来做到这一点？我在正确的轨道上吗？ （我会不断试验和更新这篇文章，直到我弄明白为止）

详情：

我有一个视图模型，它是资源和用户之间的连接表。

它具有标准时间戳：created_at 和 updated_at

我想获取用户提交的资源所花费的时间总和。

在我的资源模型中：

has_many :views, :class_name => 'UserResourceView'

然后是视图：

class UserResourceView < ActiveRecord::Base
  attr_accessible :resource_id, :user_id
  belongs_to :resource
  belongs_to :user
end

和用户：

class User < ActiveRecord::Base
  has_many :resources
  has_many :resource_views, :class_name => 'UserResourceView'
  has_many :viewed_resources, :through => :resource_views, :source => :resource
...
end

更新：

u.resources.joins(:views).group('resources.id').select('sum(views.created_at) as s_ca, sum(views.updated_at) as s_ua, s_ca - s_ua as diff').first

  Resource Load (0.7ms)  SELECT sum(views.created_at) as s_ca, sum(views.updated_at) as s_ua, s_ca - s_ua as diff FROM "resources" INNER JOIN "user_resource_views" ON "user_resource_views"."resource_id" = "resources"."id" WHERE "resources"."user_id" = 1 GROUP BY resources.id LIMIT 1
ActiveRecord::StatementInvalid: PG::Error: ERROR:  missing FROM-clause entry for table "views"

LINE 1: SELECT  sum(views.created_at) as s_ca, sum(views.updated_at)...
                    ^
: SELECT  sum(views.created_at) as s_ca, sum(views.updated_at) as s_ua, s_ca - s_ua as diff FROM "resources" INNER JOIN "user_resource_views" ON "user_resource_views"."resource_id" = "resources"."id" WHERE "resources"."user_id" = 1 GROUP BY resources.id LIMIT 1

更新 2：

u.resources.joins(:user_resource_views).group('resources.id').select('sum(user_resource_views.created_at) as s_ca, sum(user_resource_views.updated_at) as s_ua, s_ca - s_ua as diff')
ActiveRecord::ConfigurationError: Association named 'user_resource_views' was not found; perhaps you misspelled it?

更新 3（更多信息）：

来自我的 schema.rb 文件：

create_table "user_resource_views", :force => true do |t|
    t.integer  "resource_id"
    t.integer  "user_id"
    t.datetime "created_at",  :null => false
    t.datetime "updated_at",  :null => false
  end

Answer 1

这应该可以工作并最大限度地减少查询

@user.resources.joins(:views)
.group('resources.id')
.select('sum(views.created_at) as s_ca, sum(views.updated_at) as s_ua, s_ca - s_ua as diff') 

我不确定您是否可以对查询进行减号，但仍然没有，这是一种更好的方法