Ruby String to Class Name

Question

I am trying to create a new class to that will inherit from ActiveRecord::Base the class needs to be dynamically generated from a string

"general_systems".camelize.singularize = Class.new < ActiveRecord::Base

However I keep getting the error:

undefined method `singularize=' for "GeneralSystems":String

I've also tried to constantize the string

>> foo = "general_systems".camelize.singularize
=> "GeneralSystem"
>> foo.constantize
NameError: uninitialized constant GeneralSystem
    from /var/lib/gems/1.9.1/gems/activesupport-3.0.5/lib/active_support/inflector/methods.rb:124:in `block in constantize'
    from /var/lib/gems/1.9.1/gems/activesupport-3.0.5/lib/active_support/inflector/methods.rb:123:in `each'
    from /var/lib/gems/1.9.1/gems/activesupport-3.0.5/lib/active_support/inflector/methods.rb:123:in `constantize'
    from /var/lib/gems/1.9.1/gems/activesupport-3.0.5/lib/active_support/core_ext/string/inflections.rb:43:in `constantize'
    from (irb):4
    from /usr/bin/irb:12:in `<main>'
>> foo.constantize = Class.new
NoMethodError: undefined method `constantize=' for "GeneralSystem":String
    from (irb):5
    from /usr/bin/irb:12:in `<main>'

Any help would be greatly appreciated.

you are tyring to call a method "general_systems".camelize.singularize=(Class.new) which is not there. what exactly are you trying to do? — Surya, Mar 14 '11 at 17:42
Judging by one of your comments on someone's answer, you already solved the problem with their solution. Perhaps you could select an answer now? — Aaa, Mar 15 '11 at 23:06

score 38 · Answer 1 · edited Apr 19 '16 at 20:35

38

If you're using Rails, it provides a method called #constantize that will work:

irb(main):017:0> Object.const_get 'House::Owns'
NameError: wrong constant name House::Owns

'House::Owns'.constantize
=> House::Owns

edited Apr 19 '16 at 20:35

Jason Kim

18,102
13
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105

answered Dec 30 '12 at 00:12

Peter Ehrlich

6,969
4
49
65

2

I get: `1.9.3p392 :019 > 'House::Owns'.constantize NoMethodError: undefined method `constantize' for "House::Owns":String from (irb):19 from /Users/ryan/.rvm/rubies/ruby-1.9.3-p392/bin/irb:16:in
'`
– ethicalhack3r Sep 17 '13 at 12:16
6

#constantize is a Rails method. http://api.rubyonrails.org/classes/ActiveSupport/Inflector.html#method-i-constantize – Gav Mar 09 '14 at 17:00

score 33 · Answer 2 · answered Mar 14 '11 at 17:45

33

Something like this?

>> Object.const_set("general_systems".classify, Class.new)
=> GeneralSystem
>> GeneralSystem.new
=> #<GeneralSystem:0x105b0f738>

answered Mar 14 '11 at 17:45

Michael Kohl

66,324
14
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4

this is an activesupport idiom – shadowbq May 31 '15 at 22:54

score 13 · Answer 3 · answered Mar 14 '11 at 17:50

13

klazz = Class.new(ActiveRecord::Base) do
  def do_something_fun(param1)
    param1.have_fun!
  end
end

klazz_name = "general_systems".singularize.classify
Object.const_set(klazz_name, klazz)

answered Mar 14 '11 at 17:50

yfeldblum

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score 8 · Answer 4 · answered Mar 14 '11 at 17:45

Look at this example from "The Book Of Ruby", included in the Ruby 1.9 installer.

puts("What shall we call this class?> ")
className = gets.strip().capitalize()
Object.const_set(className,Class.new)
puts("I'll give it a method called > 'myname'" ) 
className = Object.const_get(className)
className::module_eval{
  define_method(:myname){ 
    puts("The name of my class is '#{self.class}'" ) 
 } }
 x = className.new x.myname

score 5 · Answer 5 · answered Dec 14 '18 at 23:42

5

Given:

class Banana
end

You can fetch the class in plain Ruby with:

Object.const_get 'Banana'
=> Banana

or if you're using Rails:

'Banana'.constantize
=> Banana

answered Dec 14 '18 at 23:42

Connor

796
11
16

Florian Feldhaus · Answer 6 · 2012-10-06T14:21:56.677

If your string contains a namespace, you can use:

require 'rubygems'
require 'active_support/inflector'
parent=String # using String to get a self contained example
# require 'active_record'   # uncomment for ActiveRecord::Base as parent class
# parent=ActiveRecord::Base # uncomment for ActiveRecord::Base as parent class
namespace = 'A::B::general_systems'.split('::') 
class_name = namespace.pop 
namespace = namespace.inject(Object) do |mod, name|
  if mod.constants.collect{|sym| sym.to_s}.include? name.classify
    mod.const_get name.classify
  else
    mod.const_set name.classify, Module.new
  end
end

klass = if namespace.constants.include? class_name.classify
  namespace.const_get class_name.classify
else
  namespace.const_set class_name.classify, Class.new(parent)
end

object = klass.allocate # allocate new object of klass
object.send :initialize # initialize object
object.class
object.class.superclass

score 1 · Answer 7 · answered Mar 14 '11 at 17:43

1

try

>> "general_systems".classify
=> "GeneralSystem"

answered Mar 14 '11 at 17:43

rm-rf

1,313
1
15
24

Thank you all, I ended up with:

Object.const_set(loader.classify, Class.new(ActiveRecord::Base)) 
 db_loader = Object.const_get(loader.classify) 
 db_loader.create(input_system)

And that works very nicely. – Telmo Mar 14 '11 at 18:25

1
@Telmo: For future reference, use backticks rather than `
```
`.
```
– Andrew Grimm Mar 14 '11 at 22:12

Ruby String to Class Name

7 Answers7

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