How to make use of "username or Email", to login in devise?

I found a way to customize devise to use username to login instead of email address.
But actually I want to use combination :-

Enter username or Email:   <text-box>
Enter your password    :   <text-box>

So how to achieve this ?

Actually we need to modify devise gem's configuration to support this.
for now I know that modifying the "/config/initializers/devise.rb" file as

config.authentication_keys = [ :username ] (edited from email to username.)

would actually enable logging in using username instead of emailid.
But can I do some thing like:-

config.authentication_keys = [ :username | :email] in the "config/initializers/devise.rb" file.

So that we devise can take username or email as authentication key at runtime depending on the input provided by user.
Thanks in advance .

Solution

It explains how to allow users to sign in using their username or email address.