[newbie]Form differs from db-table

Rails
1

Hello everyone I’m learning Ruby on rails for a school project but I
keep bumping in to problems (probably because i’m used to java, c++, c#
and php).

I have a simple table in my database and I have a simple form wich
differs from that table. But after some altering of the form values i
have to send the right values to de database.

ok lets post some code so you will understand:

database table:

class CreateUsages < ActiveRecord::Migration
  def self.up
    create_table :Usages do |table2|
      table2.column :User_id, :integer
      table2.column :Usage, :float
    end
  end

  def self.down
    drop_table :Usages
  end
end

the form:

<h1>New usage</h1>

<%= start_form_tag :action=> "create" %>

<%= error_messages_for 'usage' %><br/>

<label for="usage_kilometers">Kilometers</label><br/>
<%= text_field "usage", "kilometers", :size => 10 %><br/>

<label for="usage_liters">Liters</label><br/>
<%= text_field "usage", "liters", :size => 10 %><br/>

<%= submit_tag "Add" %>

<%= end_form_tag %>

piece of the controller:

  def create
    @usage = Usage.new(params[:usage])
    @user=session[:user]
    Usage.calculate(@usage.kilometers, @usage.liters, 10)

    if request.post?
      if @usage.save
        Usage.calculate(@usage.kilometers, @usage.liters, 10)
        flash[:notice] = 'Usage was successfully created.'
        redirect_to :action => 'list'
      else
        render :action => 'new'
      end
    end
  end

the model

require 'digest/sha1'

class Usage < ActiveRecord::Base

  attr_accessor :kilometers
  attr_accessor :liters
  attr_accessor :user

  validates_length_of :kilometers, :within => 1..10
  validates_length_of :liters, :within => 1..10
  validates_presence_of :kilometers, :liters

  def self.calculate(kilometers, liters, user)
    usage = Usage.new
    usage.User_id = user
    usage.Usage = (kilometers.to_i / liters.to_i)
    usage.save
  end
end

The problem is that every entry my form makes in de database contains
NULL values… instead of the user_id wich i set to an hard 10. or the
usage value wich is calculated…

I am stuck at this part for some time and I experimented a lot… this
code is the closesd I got.