Setting a field to read-only in Rails after it's created

You can add custom validation method to your model:

# Project model
validate :forbid_changing_name, on: :update

private

def forbid_changing_name
  errors.add :name, "can not be changed!" if self.name_changed?
end

How about setting it to read only? The attr_readonly method allows you to do it, see:

API Docs