Keep the current _timestamp in UPDATE document request

How can I keep the same _timestamp value in an update request?
I don't want it to change because of an update in the document.

I've tried something like this in the update script:

ctx._timestamp = ctx._timestamp

But it doesn't work (can't find ctx._timestamp). Is there a way to access
the current _timestamp to set it to ctx._timestamp?

Thanks

Felipe Hummel

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