why I can't update data, but I can insert data into new model

why I can't update data, but I can insert data into new model. like this: if File.find_by_id(id) file=File.find_by_id(id) else file=File.new

file.name=name file.save

there is a problem, when there don't find id, there is no problem, But when find id, update the field, there is Error, there said: undefined method 'name=' why? who can help me? Thanks!!