Ruby on Rails

I would like to have a form in which some of the fields are linked to model and some fields are not. When I use form_for tag it binds to a particular model and I cannot create form fields that aren't correspond to one of the column name of the table (Model). Hence what I think is I should use form_tag instead of form_for so that it contains whatever the form fields it want but one thing I can't find by googling is how can I connect specific fields to a model leaving other fields free of model?

In other words, If I have 5 text_fields in a form how can I link only 3 text_fields to a model and leave other 2 fields free?

Thank you.