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Platform: Mac OSX 10.6

In my terminal, i start the Ruby console with "rails c"

While following the Ruby on Rails 3 tutorial to build a class:

class Word < String def palindrome? #check if a string is a palindrome self == self.reverse end end

i get the error message:

TypeError: superclass mismatch for class Word from (irb):33 from /Users/matthew/.rvm/gems/ruby-1.9.2-p180@rails3tutorial/gems/railties-3.0.5/lib/rails/commands/console.rb:44:in `start' from /Users/matthew/.rvm/gems/ruby-1.9.2-p180@rails3tutorial/gems/railties-3.0.5/lib/rails/commands/console.rb:8:in `start' from /Users/matthew/.rvm/gems/ruby-1.9.2-p180@rails3tutorial/gems/railties-3.0.5/lib/rails/commands.rb:23:in `<top (required)>' from script/rails:6:in `require' from script/rails:6:in `<main>'

The tutorial shows that it has no problem and i know the code is fine; I've searched other related questions, but they all involved migrating from Ruby 2 to 3 or erb vs eruby.

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6 Answers 6

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95

You already have a Word class defined elsewhere. I tried within a Rails 3 app but was not able to replicate.

If you have not created a second Word class yourself, it is likely one of your Gems or plugins already defines it.

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4 Comments

Thanks, i ended up restarting the console. Earlier, i did use a different class name, yet got the same error.
Thanks for the hint about gems or plugins probably aready defining the class. Had the problem in a JRuby 1.7.1 application in my PagesController. Renaming it solved the issue.
But the problem in not the existence of the class itself, but that it has different parent class.
restarting the console worked for me as well @Matthew
28

This can also happen as such:

# /models/document/geocoder.rb class Document module Geocoder end end # /models/document.rb require 'document/geocoder' class Document < ActiveRecord::Base include Geocoder end

The require loads Document (which has a superclass of Object) before Document < ActiveRecord::Base (which has a different superclass).

I should note that in a Rails environment the require is not usually needed since it has auto class loading.

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23

I had the problem with a Rails 4 application. I used concerns under the user namespace.

class User module SomeConcern end end

In development everything worked fine but in production (I guess because of preload_app true) I got the mismatch error. The fix was pretty simple. I just added an initializer:

require "user"

Cheers!

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6 Comments

That's the most frustrating thing for me! Wish I could upvote it more!
I also have this problem, can you tell me where require 'user' added?
@JeskTop just add a new file to config/initializers with just that single file. Initializers run before the whole application gets loaded and make sure that the model class gets loaded first.
@2called-chaos Thanks, but it still not work for me. I add a question, but no one can help me fix this problem. stackoverflow.com/questions/30795339/…
This solved my problem, however I am not sure why I have this problem. I have 2 models order.rb and notification.rb and nowhere in my app is any other Order of Notification class
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8

Sometimes we 'open class' without us knowing. For example with some deep module nesting:

# space_gun.rb class SpaceGun << Weapon def fire Trigger.fire end end # space_gun/trigger.rb class SpaceGun class Trigger end end

When we define trigger, we open the existing SpaceGun class. This works. However if we load the two file in the reverse order, the error would be raised, because we would define a SpaceGun class first, but is not a Weapon.

Sometimes we make this mistake because we explicitly require sub module (e.g. trigger) from the parent class. Which means the class definition will be done the reverse order, causing this issue.

# surely nothing can go wrong if we require what we need first right? require 'space_gun/trigger' class SpaceGun << Weapon def fire Trigger.fire end end # BOOM

Either

  1. rely on auto loading
  2. always put inheritance to every open class occurrence.
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I believe << is a typo.
5

I had this same problem right now. Basically that means that Word is defined as a class elsewhere and my guess is that it's on the rail-ties gem. Just change Word to Word2 and it should work fine on the tutorial.

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1

Depending on your specific case, any of the already-provided answers could be your solution.

To find out quickly and easily, temporarily set config.eager_load = true and start your rails console. It should immediately fail with a more explicit error message, pointing at the right file and line number.

Don't forget to set config.eager_load back to its original value after.

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2 Comments

In my case eager loading actually solved the problem. I did not get a more detailed error message after enabling it. Interesting… (Maybe it lead to both occurrences being loaded in reverse order and one occurrence did not explicitly declare a superclass?)
@einjohn This means you need to check on your inclusions, you are certainly missing one somewhere

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