[Rails] unique boolean flag

[Rodrigo Alvarez Fernández]

Hi!

I have a field "default", and i wish to validate that only one of the
records is true, the remaining must be false.
If there are no records, the first one created must be default=true.
Apart from that, when any new/updated record is set to default=true, the
remaining ones must be default=false.

How do you accomplish this?

Thanks in advance,
Rodrigo.
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