devise.rb

################################################################
#  For views/migrations etc. check http://tinyurl.com/3xfx3zm  #
################################################################

# File : RAILS_APP/config/initializers/devise.rb
# Change the following only. Rest can stay same
# NOTE : You must use devise master or any version released after Mar 13, 2011 to get everything mentioned here working.
config.authentication_keys = [ :login ]
config.confirmation_keys = [ :login ]
config.unlock_keys = [ :login ]

user.rb

###############################################################
# IMPORTANT: You need meta_where gem for where clause to work #
# Tinker accordingly if you don't want to use meta_where      #  
###############################################################

# File : RAILS_APP/app/models/user.rb
# Your user model. Select appropriate model depending on your devise config

# To facilitate username or email login
attr_accessor :login

# Setup accessible (or protected) attributes for your model
# NOTE the :login here.
attr_accessible :login, :username, :email, :password, :password_confirmation, :remember_me


# Essential functions
protected

def self.find_for_database_authentication(conditions)
  login = conditions.delete(:login)
  u = self.arel_table
  where(conditions).where(u[:username].eq(login).or(u[:email].eq(login))).first
end

def self.find_or_initialize_with_errors(required_attributes, attributes, error=:invalid)
  case_insensitive_keys.each { |k| attributes[k].try(:downcase!) }

  attributes = attributes.slice(*required_attributes)
  attributes.delete_if { |key, value| value.blank? }

  if attributes.size == required_attributes.size
    if attributes.has_key?(:login)
      login = attributes.delete(:login)
      record = find_record(login)
    else  
      record = where(attributes).first
    end  
  end  

  unless record
    record = new

    required_attributes.each do |key|
      value = attributes[key]
      record.send("#{key}=", value)
      record.errors.add(key, value.present? ? error : :blank)
    end  
  end  
  record
end

def self.find_record(login)
  u = self.arel_table
  where(u[:username].eq(login).or(u[:email].eq(login))).first
end