當不在parens里面時，Ruby三元條件似乎被忽略了？

Question

我試圖理解為什么將這個if-then-else行轉換為三元表達式（ ?: :)不起作用。 我知道它可能不是最合適的代碼，但我想了解發生了什么。

   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傳遞了測試，並且一切都很好，但是當我試圖使它像這樣的三元表達式時，它無法像我預期的那樣工作。

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

這讓我失敗了：

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 nil ，它怎么能做三元表達式的真正部分？

如果我把parens放在這樣的條件下，它可以正常工作：

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

我認為它必須是某種優先級的東西，但我只是看不到它。 非常感謝您花時間回復並幫助我學習。

Answer 1

這是一個優先事項。 ?:優先級高於= ，允許您根據條件進行分配：

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

可以理解地解析為

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

如果要在條件內分配，則需要括號：

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

在你的情況下，

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

它將首先評估Artist.find_by並根據該決定條件; 如果它發現的東西，它試圖評估@songs = a.songs ，但是a 尚未分配 。 a正在等待條件的結果，並且在條件完成時將接收@songs = a.songs的值或redirect_to的值。

從風格上來說，我寧願把這個留下來， if ... then ... else ... end ，因為你沒有使用結果?: 。 當條件返回的值相關時，通常使用三元條件運算符，並且兩個分支都是相對簡單的表達式。 我會使用if ... then ... else ... end如果沒有使用返回值（如代碼中），或者如果分支包含多個簡單表達式，特別是多線或多線聲明：

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