Rails devise error “NoMethodError … merge”

Question

I was trying to make a User model with devise (so far in my application I have not had a problem with making Models or adding attributes). I run

rails g devise user

Which creates this:

  invoke  active_record
  create    db/migrate/20140930235224_devise_create_users.rb
  create    app/models/user.rb
  invoke    test_unit
  create      test/models/user_test.rb
  create      test/fixtures/users.yml
  insert    app/models/user.rb
  route  devise_for :users

Then I try to run

rake db:migrate

Which gives me the following

rake aborted!
NoMethodError: undefined method `merge!' for #<ActionDispatch::Routing::Mapper::Scope:0x9fc73a4>
/home/user/Desktop/MyApp/config/routes.rb:2:in `block in <top (required)>'
/home/user/Desktop/MyApp/config/routes.rb:1:in `<top (required)>'
/home/user/Desktop/MyApp/config/environment.rb:5:in `<top (required)>'
Tasks: TOP => db:migrate => environment
(See full trace by running task with --trace)

What could be the problem?

Line 2 for routes is:

devise_for :users

And line 5 for environment is:

Rails.application.initialize!

Answer 1

Check what devise version you have installed by running:

bundle show devise

With the latest version of Rails 4.2.0, devise will need to be upgraded to 3.4.0 or higher. To do so, specify in your Gemfile:

gem 'devise', '~> 3.4.0'

Then run:

bundle update devise

Answer 2

It looks like this may be a problem with Devise and the latest version of Rails. Have a look at these issues on Devise Github:

• Issue 3158
• Issue 3206
• Issue 3151

This blog post explains that there is a branch with the fix, however, if you are going to be using this in production you should probably wait until Devise merge the fix into master.

Answer 3

Updating the version of devise from 3.3.0 to 3.4.0 in my Gemfile and running bundle update devise worked like magic for me. Im running rails 4.2.0.