Rails：从特定列中获取数据？

Question

我想只从特定列中提取数据，但返回的是整个记录。

我走进我的控制台，试图看看引擎盖下发生了什么，但结果让我困惑。

我怎样才能返回每个记录的状态？

1.9.3p194 :001 > company = Company.first 
Company Load (0.1ms) SELECT "companies".* FROM "companies" LIMIT 1 
=> #<Company id: 1, name: "Supreme WIndows", created_at: "2012-06-24 04:12:02", updated_at: "2012-06-24 04:12:02"> 
1.9.3p194 :002 > company.requests 
Request Load (0.1ms) SELECT "requests".* FROM "requests" WHERE "requests"."requestable_id" = 1 AND "requests"."requestable_type" = 'Company' 
=> [#<Request id: 1, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 01:25:11", updated_at: "2012-06-30 01:25:11", profile_id: nil>, # 
    <Request id: 2, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 01:28:16", updated_at: "2012-06-30 01:28:16", profile_id: nil>, # 
    <Request id: 3, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 01:32:55", updated_at: "2012-06-30 01:32:55", profile_id: nil>, # 
    <Request id: 4, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 02:46:07", updated_at: "2012-06-30 02:46:07", profile_id: nil>] 
1.9.3p194 :003 > company.requests.status 
NoMethodError: Request Load (0.2ms) SELECT "requests".* FROM "requests" WHERE "requests"."requestable_id" = 1 AND "requests"."requestable_type" = 'Company' 
undefined method `status' for #<ActiveRecord::Relation:0x007fdbd57e1ba8> 
from /Users/Aaron/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.3/lib/active_record/relation/delegation.rb:45:in `method_missing' 
from /Users/Aaron/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.3/lib/active_record/associations/collection_proxy.rb:100:in `method_missing' 
from (irb):3 
from /Users/Aaron/.rvm/gems/ruby-1.9.3-p194/gems/railties-3.2.3/lib/rails/commands/console.rb:47:in `start' 
from /Users/Aaron/.rvm/gems/ruby-1.9.3-p194/gems/railties-3.2.3/lib/rails/commands/console.rb:8:in `start' 
from /Users/Aaron/.rvm/gems/ruby-1.9.3-p194/gems/railties-3.2.3/lib/rails/commands.rb:41:in `<top (required)>' 
from script/rails:6:in `require' 
from script/rails:6:in `<main>' 
1.9.3p194 :004 > company.requests.first.status 
=> nil 
1.9.3p194 :005 > company.requests.each do |request| 
1.9.3p194 :006 >  request.status 
1.9.3p194 :007?> end 
=> [#<Request id: 1, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 01:25:11", updated_at: "2012-06-30 01:25:11", profile_id: nil>, # 
    <Request id: 2, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 01:28:16", updated_at: "2012-06-30 01:28:16", profile_id: nil>, # 
    <Request id: 3, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 01:32:55", updated_at: "2012-06-30 01:32:55", profile_id: nil>, # 
    <Request id: 4, status: nil, requestable_id: 1, requestable_type: "Company", created_at: "2012-06-30 02:46:07", updated_at: "2012-06-30 02:46:07", profile_id: nil>] 
1.9.3p194 :008 > 

全部为MCV代码是在以下问题 Rails: How to create polymorphic relationship/friendship model

我已经根据它的瑞恩·贝茨多态性associtions代码 https://github.com/railscasts/154-polymorphic-association-revised/tree/master/blog-after

Answer 1

的Rails 3.1或更高版本引入了pluck方法完全相同这company.requests.pluck(:status)。这将查询数据库的状态并返回一个数组。

在旧版本中，用户可以使用company.requests.select(:status).map(&:status)，在这里您将为每个请求实例化一个模型，然后只需提取状态，这会花费更多的代价。