How To: Migration legacy database

migrate a legacy user database to devise, which is using password MD5 hexdigest without salt, put this code in initializers:

module Devise
  module Models
    DatabaseAuthenticatable.class_eval do      
      def valid_password_with_legacy?(password)
        if self.legacy_password_hash.present?
          if ::Digest::MD5.hexdigest(password).upcase == self.legacy_password_hash
            self.password = password
            self.legacy_password_hash = nil
            self.save!
            return true
          else
            return false
          end
        else
          return valid_password_without_legacy?(password)
        end
      end

      alias_method_chain :valid_password?, :legacy
    end

    Recoverable.class_eval do
      def reset_password_with_legacy!(new_password, new_password_confirmation)
        self.legacy_password_hash = nil
        reset_password_without_legacy!(new_password, new_password_confirmation)
      end

      alias_method_chain :reset_password!, :legacy
    end
  end
end

---

Josevalim suggested a much better solution in (issue #1858), just override valid_password? and reset_password? and use 'super' in your devise model

    class User
      def valid_password?(password)
        if self.legacy_password_hash.present?
          if ::Digest::MD5.hexdigest(password).upcase == self.legacy_password_hash
            self.password = password
            self.legacy_password_hash = nil
            self.save!
            true
          else
            false
          end
        else
          super
        end
      end

      def reset_password!(*args)
        self.legacy_password_hash = nil
        super
      end
    end