I would like to use formtastic to create form but I don't have a model associated with that (login form with username, password and openid URL).
Of course I could create a model to do that but that model was just a hack without any useful code in it.
You can pass a string instead of a model, which will be used to generate the field names:
<% semantic_form_for 'user', :url => 'login' do |f| %>
<% f.inputs :name => 'Login Details' do %>
<%= f.input :username %>
<%= f.input :password %>
<% end %>
<% end %>
Which will produce something like:
<form action="/login" class="formtastic user" method="post"><div style="margin:0;padding:0;display:inline"><input name="authenticity_token" type="hidden" value="E/NksKRd7Twh4nGp1Qc8jBQNfqYDn8gg6sWdTdCtl+g=" /></div>
<fieldset class="inputs"><legend><span>Login Details</span></legend><ol>
<li class="string required" id="user_username_input"><label for="user_username">Username<abbr title="required">*</abbr></label><input id="user_username" name="user[username]" size="50" type="text" /></li>
<li class="password required" id="user_password_input"><label for="user_password">Password<abbr title="required">*</abbr></label><input id="user_password" name="user[password]" size="50" type="password" /></li>
</ol></fieldset>
</form>
But you will need to be more specific with your options as formtastic won't be able to work out what types of fields it should use, all will default to textfields (although it automatically makes fields named like password password type fields).
A better way to do this though would be with a sessions model, have a look at the way authlogic (http://github.com/binarylogic/authlogic) works for more info.