The things happen while upgrading from Rails 3.2 to 4

The things happen while upgrading from Rails 3.2 to 4

Oops! Here comes the deprecation warning.  It's the "secret_key_token".
The secret_key_base is not required, however,  you will get a deprecation warning if this line is missing:

DEPRECATION WARNING: You didn't set config.secret_key_base.

There is a new value for that initializer in config/initializers/secret_token.rb

Myapp::Application.config.secret_token = '6839bw..' # your current token
Myapp::Application.config.secret_key_base ='db6s..' # secret value, this needs to be added

In order to generate a random secret_key value, you can use rake task:

rake secret

Just copy the value generated to the second line above, then restart the server and it's done!

More details: Upgrading to Rails 4.0 from Rails 3.2 – Test case