Rails - Mar 2008 - Conditional validation?

Hi, I have followed the tutorial in the AWDWR book for uploading an
image.  This involves adding the following field to the form.
<%= f.file_field :uploaded_picture %>

The model then goes to work on the data to store it in the database,
with the following:

validates_format_of :content_type,
:with => /^image/,
:message => "-- you can only upload pictures"

def uploaded_picture=(picture_field)
self.name = base_part_of(picture_field.original_filename)
self.content_type = picture_field.content_type.chomp
self.data = picture_field.read
end

def base_part_of(file_name)
File.basename(file_name).gsub(/[^\w._-]/, '''' )
end

Now, all of this works fine, but I would like the picture upload to be
an optional feature, but if the user fills in the form and leaves the
field blank, the following error is displayed

undefined method `original_filename'' for "":String

I''ve tried adding "unless picture_field.empty/nil? to the
uploaded_picture action, but I can''t get anything that works for both
an
image being supplied and no image being supplied.
Can anyone suggest a way to get around this?  Thanks.
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Dan Smith wrote:
> validates_format_of :content_type,
> :with => /^image/,
> :message => "-- you can only upload pictures"
> 
> I''ve tried adding "unless picture_field.empty/nil? to the
Try "unless picture_field.blank?" and add ":allow_blank =>
true" to the
validation line.

If you are using Rails earlier than version 2, then make that 
":allow_nil => true", though I''m not sure if that will be
as successful.

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Thanks, that seems to have worked:)
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