Update database field in ruby on rails

I’m updating a database hashing algorithm.

My current system runs on md5 and I want to change it into BCrypt

My problem is when an old user(users whos password hashed in md5) is
login with his old password I want to automatically change the password
to BCrypt+salt in database.

       if // check if the password stored in bcrypt
        salt = IDA::Config.get_configuration('salt')
        hash_password = BCrypt::Password.new(hash)

(BCrypt::Password.create(salt[‘salt_value’]+password) ==
(salt[‘salt_value’]+password)) ? true : false

      else // for users who's password encrypted in md5

        salt = IDA::Config.get_configuration('salt') // i"m getting

a salt here
BCrypt::Password.create(salt[‘salt_value’]+password) // Im
getting a salted bcryptted password and I tried to put this into db
manually and try to login it works perfectly
// I want to write this new salted password into db once
the user is authenticated with his old password
return (Digest::MD5.hexdigest(password) == hash) ? true :

I want to write this in model.Any help will be greatly appreciated.