当不在括号内时，Ruby 三元条件似乎被忽略了？

Question

我试图理解为什么将此 if-then-else 行转换为三元表达式 (?:) 不起作用。我知道这可能不是最合适的代码，但我想了解发生了什么。

   if a = Artist.find_by(id: params[:artist_id]) then @songs = a.songs else redirect_to(artists_path, alert: "Artist not found") end

上面的if-then-else 通过了测试，一切都很好，但是当我尝试将其设为像这样的三元表达式时，它无法按预期工作。

  a = Artist.find_by(id: params[:artist_id]) ? @songs = a.songs : redirect_to(artists_path, alert: "Artist not found")

这给了我以下失败：

1) songs when nested under artists /artists/:artist_id/songs displays the songs with valid artist
     Failure/Error: a = Artist.find_by(id: params[:artist_id]) ? @songs = a.songs : redirect_to(artists_path, alert: "Artist not found")

     NoMethodError:
       undefined method `songs' for nil:NilClass
     # ./app/controllers/songs_controller.rb:4:in `index'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/etag.rb:24:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/conditionalget.rb:25:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/head.rb:13:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/session/abstract/id.rb:225:in `context'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/session/abstract/id.rb:220:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/railties-4.2.5/lib/rails/rack/logger.rb:38:in `call_app'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/railties-4.2.5/lib/rails/rack/logger.rb:20:in `block in call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/railties-4.2.5/lib/rails/rack/logger.rb:20:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/methodoverride.rb:22:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/runtime.rb:18:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/lock.rb:17:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/sendfile.rb:113:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/railties-4.2.5/lib/rails/engine.rb:518:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/railties-4.2.5/lib/rails/application.rb:165:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/urlmap.rb:66:in `block in call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/urlmap.rb:50:in `each'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-1.6.4/lib/rack/urlmap.rb:50:in `call'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-test-0.6.3/lib/rack/mock_session.rb:30:in `request'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-test-0.6.3/lib/rack/test.rb:244:in `process_request'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/rack-test-0.6.3/lib/rack/test.rb:58:in `get'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/capybara-2.5.0/lib/capybara/rack_test/browser.rb:60:in `process'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/capybara-2.5.0/lib/capybara/rack_test/browser.rb:35:in `process_and_follow_redirects'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/capybara-2.5.0/lib/capybara/rack_test/browser.rb:21:in `visit'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/capybara-2.5.0/lib/capybara/rack_test/driver.rb:42:in `visit'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/capybara-2.5.0/lib/capybara/session.rb:232:in `visit'
     # /home/dapawn/.rvm/gems/ruby-2.3.3/gems/capybara-2.5.0/lib/capybara/dsl.rb:51:in `block (2 levels) in <module:DSL>'
     # ./spec/features/songs_spec.rb:13:in `block (4 levels) in <top (required)>'

如果a 是nil，它怎么能做三元表达式的真正部分？

如果我在这样的条件周围加上括号，它就可以正常工作：

   (a = Artist.find_by(id: params[:artist_id])) ? @songs = a.songs : redirect_to(artists_path, alert: "Artist not found")

我认为它必须是某种优先级的东西，但我就是看不到它。提前感谢您抽出时间回复并帮助我学习。

Answer 1

这是一个优先的事情。 ?: 的优先级高于=，允许你根据条件分配：

animal = is_dog ? "dog" : "not a dog"

可以理解为

animal = (is_dog ? "dog" : "not a dog")

如果要在条件内赋值，则需要括号：

(animal = find_animal("dog")) ? "found dog" : "no dog"

在你的情况下，

a = Artist.find_by(id: params[:artist_id]) ?
    @songs = a.songs :
    redirect_to(artists_path, alert: "Artist not found")

它将首先评估Artist.find_by 并根据它决定条件；如果它发现了什么，它会尝试评估@songs = a.songs，但a 尚未分配。 a 正在等待条件的结果，当条件完成时，将收到 @songs = a.songs 的值或 redirect_to... 的值。

从风格上来说，鉴于您没有使用?: 的结果，我更愿意将其保留为if ... then ... else ... end。三元条件运算符通常用于条件返回的值是相关的，并且两个分支都是相对简单的表达式。如果不使用返回值（例如在您的代码中），或者如果分支包含的不仅仅是一个简单的表达式，我会使用 if ... then ... else ... end，尤其是在多行或多语句时：

foo =
  if bar
     do_something
     do_something_else(bar)
     final_result
  else
     do_another_thing
     skip_something_else
     the_other_final_result
  end