The attributes returned by "form_for" issue

Hello, everybody:
   I have made a "form_for" in a view returns the attributes value of
a "user" model, such as user.login,,, ect.
   However, now I wanna add a new , e.g. "code", into the form_for
tag, and return it to next action. The question is this attribute
doesn't belong to "user" model, it is simply a string i would like to
deliver to next action. Once I try this , it errors: undefined
method `number' for #<User:0xb5924cac>.
   Is the "form_for" tag merely able to convey model's attribute

Try using <%= text_field_tag “number” %> instead of f.text_field

Thanks! I used to conside text_filed_tag should be only used in