Using form_tag with a bit of rails 'magic'?

I’m trying to follow an example from ‘Agile Web Development with Rails’ (3rd ed) which shows how to upload an image file, except that I’m trying to upload a csv file.

The ‘magic’ that the example shows is how the file_field helper method can say “file_field(“uploaded_file”)” when there is actually no “uploaded_file” column in the upload table, because the following method can be put in the model:

def uploaded_file=(upload_field)
self.filename = base_part_of(file_field.original_filename)
# is here is where the uploaded file can be parsed?
FasterCSV.foreach(file.path,:headers=>“first_row”, :col_sep=>"\t") do |row|
row.each{|row| puts “row: #{row.inspect}”}
end
end

def base_part_of(file_name)
File.basename(file_name)
end

However, this method never seems to fire. Instead, when I upload the file, I get the error:

ERROR: null value in column "filename" violates not-null constraint

which, of course, would happen if a record is inserted without assigning the filename to the upload object.

My code is shown below; would appreciate if anyone can tell me why my uploaded_file method is not firing:

My model:
<% form_tag (:action=> ‘create’, :multipart => true) do %>
<%= file_field_tag ‘uploaded_file’ %>

<%= submit_tag "Upload" %>

<% end %>

My controller:
def create
@upload = Upload.new(params[:upload])

respond_to do |format|
  if @upload.save
    flash[:notice] = 'Upload was successful.'
    format.html { redirect_to(@upload) }
    format.xml  { render :xml => @upload, :status => :created, :location => @upload }
  else
    format.html { render :action => "new" }
    format.xml  { render :xml => @upload.errors, :status => :unprocessable_entity }
  end
end

end

My model:
require ‘fastercsv’

def new
@upload = Upload.new
end

def uploaded_file=(upload_field)
self.filename = base_part_of(file_field.original_filename)
# is here is where the uploaded file can be parsed?
FasterCSV.foreach(file.path,:headers=>“first_row”, :col_sep=>"\t") do |row|
row.each{|row| puts “row: #{row.inspect}”}
end
end

def base_part_of(file_name)
File.basename(file_name)
end

Your setter needs to be same name as the field that rails is looking for.