<div dir="ltr"><br><br><div class="gmail_quote">On Sun, Apr 17, 2011 at 7:37 AM, Ian Duncan <span dir="ltr"><<a href="mailto:iand675@gmail.com">iand675@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div style="word-wrap:break-word"><div><div></div><div class="h5"><br><div><div>On Apr 16, 2011, at 10:53 PM, Michael Snoyman wrote:</div><br><blockquote type="cite"><div dir="ltr"><br><br><div class="gmail_quote">On Sun, Apr 17, 2011 at 6:27 AM, Ian Duncan <span dir="ltr"><<a href="mailto:iand675@gmail.com" target="_blank">iand675@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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?<br>
<br></blockquote><div><br></div><div>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.</div>
<div><br></div><div>Michael </div></div></div>
</blockquote></div><br></div></div><div>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.</div>
</div></blockquote></div><br><div>I'll tell you the theory for requiring the Update keyword (and Eq, Lt, Gt, Asc, Desc, ...). In theory, backends can optimize for different use cases. The prime example I give is that a SQL backend could automatically add indices if it knows that a field will be sorted on. (We don't do this currently, but I'll consider adding it in the future.) It's entirely possible that there are backends for which a more optimal structure can be set up when only certain fields can be updated.</div>
<div><br></div><div>That's the theory. Maybe in practice this was all a bad idea, feel free to let me know how you feel either way ;).</div><div><br></div><div>Michael</div></div>