Why does null monkeypatch remove methods in migration but not in rails console?

Question

This behavior is pretty cool because it makes it easy to write "futureproof" migrations where the migration defines the model class inline in case the class is later removed from the codebase. But how does it work?

$ rails c
pry(main)> User.methods.count
=> 686
pry(main)> class User < ActiveRecord::Base; end
=> nil
pry(main)> User.methods.count
=> 686

Here's a migration:

class FrobulateUsers < ActiveRecord::Migration
  puts User.methods.count

  class User < ActiveRecord::Base
  end

  puts User.methods.count
  ...
end

And when I run it:

$ rake db:migrate
687
666

How come the methods disappear in the migration but not in the console?

Answer 1

Messing around with this, this is the true class names...

class FrobulateUsers < ActiveRecord::Migration

  puts User.name

  class User < ActiveRecord::Base
  end

  puts User.name

end

Output

User
FrobulateUsers::User

A note, you can put this on the outside of the class...

# 20150810221740_frobulate_users.rb

class User < ActiveRecord::Base
end

class FrobulateUsers < ActiveRecord::Migration

  puts User.name

end

Output

User