I see 2 other options:<br><br>a) keep the last (longitude, latitude) pair for every receiver stored in your script, too, besides the db. When a new pair of values arrives, compare it with the current and only if it is different, send the "INSERT" query to the database.<br>
<br>b) When a new pair of values arrives, send a query (or stored procedure) that tells the database to check and "INSERT" only if the pair is different that the latest. <br><br>Pandelis Theodosiou<br><br><br><div class="gmail_quote">
On Mon, May 9, 2011 at 1:50 PM, Albert Brandl <span dir="ltr"><<a href="mailto:albert.brandl@weiermayer.com" target="_blank">albert.brandl@weiermayer.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">Hi!<br>
<br>
In the application I'm currently working on, we gather data from<br>
different sources and store them in a database, but only if the<br>
values have changed.<br>
<br>
One example is GPS information. We don't want to store hundreds of<br>
identical values that are sent by the receiver, so we have to compare the<br>
last written row with the current latitude / longitude pair.<br>
<br>
I use a subclass of NMEARreceiver class to determine the current latitude<br>
and longitude. Then I query the database using a ConnectionPool instance<br>
to get the previous values for latitude and longitude. If the result<br>
differs from the current values, a new entry is stored, again using the<br>
ConnectionPool.<br>
<br>
The problem is: How do I "connect" the current latitude and longitude to<br>
the database query? The query returns a Deferred object, but this expects<br>
a callback with only one argument, the result of the query.<br>
<br>
Storing them as instance variables of NMEAReceiver does not seem to be<br>
the right way, since I don't know how long it takes to query the database<br>
- the relevant GPS information might be overwritten by a new NMEA<br>
telegram when the connection pool finally triggers the callback<br>
<br>
One workaround I've found is to add this information to the SQL query,<br>
but this seems quite weird to me (and doesn't work in the case that the<br>
query does not return a result).<br>
<br>
A different approach would be to implement a callback with three<br>
arguments (result, latitude and longitude), and wrap it in a lambda<br>
function that fills latitude and longitude with the current values.<br>
Would this would be a better way to handle this requirement?<br>
<br>
How would you implement this?<br>
<br>
Thanks & best regards,<br>
<br>
Albert<br>
--<br>
Albert Brandl<br>
Weiermayer Solutions GmbH | Abteistraße 12, A-4813 Altmünster<br>
phone: +43 (0) 720 70 30 14 | fax: +43 (0) 7612 20 3 56<br>
web: <a href="http://www.weiermayer.com" target="_blank">http://www.weiermayer.com</a><br>
<br>
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