image_dimension_validation

process :get_geometry
def geometry
    @geometry
end

def get_geometry
    if (@file)
        img = ::Magick::Image::read(@file.file).first
        @geometry = [ img.columns, img.rows ]
    end
end

#User class

validate :validate_minimum_image_size

def validate_minimum_image_size
    geometry = self.image.geometry
    if (! geometry.nil?)
        self.width = geometry[0]
        self.height = geometry[1]
    end
    unless (self.width == 320 && self.height == 480) || (self.width == 640 && self.height == 960)
        errors.add :base, "Naughty you... iPhone designs should be 320px x 480px or 640px x 960px." 
    end
end