Rails authentication checks

I have this authenticate! method where I am trying to retrieve their account based on the subdomain, then find the user. if user is found & their password is matched, we return success, else fail in all other cases. here is the code:

def authenticate!
    account = Account.find_by(subdomain: subdomain)
    if account
      u = account.users.find_by email: params["user"]["email"]
      if u.nil?
        fail!
      else
        u.authenticate(params["user"]["password"]) ? success!(u) : fail!
      end
    else
      fail!
    end
end

Now, I could simply this further by moving subdomain into a helper method,

def subdomain
  ActionDispatch::Http::URL.extract_subdomains(request.host, 1)
end

def authenticate!
      account = Account.find_by(subdomain: subdomain)
      if account
        u = account.users.find_by email: params["user"]["email"]
        if u.nil?
          fail!
        else
          u.authenticate(params["user"]["password"]) ? success!(u) : fail!
        end
      else
        fail!
      end
end

But how to I simply these nested conditionals?

Another view on @tokland solution:

def authenticate!
  return fail! unless account = Account.find_by(subdomain: subdomain)
  return fail! unless user = account.users.find_by(email: params["user"]["email"])
  return fail! unless user.authenticate(params["user"]["password"])
  success! user
end

This intention is to be as close to "original task explained in common English" as possible. And return keyword allows me to do that.

StackExchange Code Review Q#31619, answer score: 8