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!!

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