Listing subclasses doesn't work in Ruby script/console?

问题

This works:

>> class Foo 
>>   def xyz()
>>     Foo.subclasses
>>   end
>> end
=> nil
>> class Bar < Foo
>> end
=> nil
>> class Quux < Bar
>> end
=> nil
>> Foo.new.xyz()
=> ["Quux", "Bar"]

But this doesn't. User is a class in my application.

>> User.subclasses
NoMethodError: protected method `subclasses' called for #<Class:0x20b5188>
    from [...]/vendor/rails/activerecord/lib/active_record/base.rb:1546:in `method_missing'
    from (irb):13

But this does!

>> Foo.subclasses
=> ["Quux", "Bar"]

What's going on here? How would I list the subclasses of User?

回答1:

subclasses is overridden and made protected in base.rb. See http://www.google.com/codesearch/p?hl=en&sa=N&cd=1&ct=rc#m8Vht-lU3vE/vendor/rails/activerecord/lib/active_record/base.rb&q=active_record/base.rb (line 1855 defines method subclasses, line 1757 makes it protected).

You could do the same for User as you did for Foo: add a xyz() method.

回答2:

You don't need to redeclare (as in Tim's answer) or provide a helper method (as in Rutger's answer). You just need to change the permission of the method (which, being a class method, requires some shenanigans):

class User < ActiveRecord::Base

  class <<self
    public :subclasses
  end

end

回答3:

Just to work around the access rights and not change anything, consider using the #send method that has access to private methods.

User.send(:subclasses)

回答4:

tables = ActiveRecord::Base.connection.tables {|t| t.classify.constantize rescue nil}.compact
subclasses = tables.map do |table|
  table.singularize.classify.constantize
end

来源：https://stackoverflow.com/questions/1195531/listing-subclasses-doesnt-work-in-ruby-script-console