当不在parens里面时,Ruby三元条件似乎被忽略了？

Question

我试图理解为什么将这if-then-else一行转换为三元表达式(?:)不起作用.我知道它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if-then-else通过测试,一切都很好,但是当我试图使它成为一个三元表达式时,它就不能像我预期的那样工作.

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

这让我失败了:

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)>'

如果它a是nil如何做三元表达的真正部分？

如果我把parens放在这样的条件下,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")

我认为它必须是某种优先级的东西,但我只是看不到它.非常感谢您花时间回复并帮助我学习.

Answer 1

这是一个优先事项.?:优先级高于=,允许您根据条件分配:

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

可以理解地解析为

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

如果要在条件内分配,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和决定基于此的条件; 如果它找到了什么,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,并且在条件完成时将接收...的值@songs = a.songs或值redirect_to.

从风格上来说,我宁愿把它留下来if ... then ... else ... end,因为你没有使用结果?:.当条件返回的值相关时,通常使用三元条件运算符,并且两个分支都是相对简单的表达式.if ... then ... else ... end如果不使用返回值(如代码中),或者如果分支包含多个简单表达式,我会使用,特别是如果是多行或多语句:

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