[web-devel] What does Persist's PersistUpdate datatype do?

Ian Duncan iand675 at gmail.com
Sun Apr 17 06:37:34 CEST 2011

On Apr 16, 2011, at 10:53 PM, Michael Snoyman wrote:

> On Sun, Apr 17, 2011 at 6:27 AM, Ian Duncan <iand675 at gmail.com> wrote:
> I'm fiddling around a bit with the Persist library and wondering what the PersistUpdate datatype does. From what I can gather, it looks like Update replaces the value, add will add to the current value if it is an integer or double, and subtract, multiply, and divide perform their respective operations. Is this correct, or is there something else to these types?
> Nope, that's it. In SQL, none of the addition/subtraction/multiplication/division logic is performed in Persistent, rather it's all passed off to the SQL engine. So if you try to divide two strings, the result depends on what the SQL engine allows.
> Michael 

If I may ask then, does not using the Update keyword in the quasiquoting syntax simply mean that no default implementation of update is provided? Is the rationale for this to prevent accidental modifications of fields that should remain static once a row is inserted? It seems like a somewhat cumbersome restriction.
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