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PostgreSQL on Leopard gotchas

Jan 16 by Andre

For a recent Rails project, I had to use PostgreSQL instead of the standard MySQL for the database. Setting up Postgres on Mac OS 10.5 has some quirks, which I will share with you here.

  • There are three primary ways to get PostGRES onto Leopard: a pre-built binary, MacPorts, and source. My choice was to build with source. There are several good posts on building from source:

  • I do not recommend creating a separate user to run postgres. Doing so clutters your startup screen with another user, and requires you to su to that user to start the database. Instead:

    1) install Postgres as root

    2) after install, from the command line: chown -R andre /usr/local/pgsql/data. The -r flag changes the ownership recursively. Obviously, substitute your own username for "andre" in the example.

  • Since I don't use PostGres all the time, I don't want it to be started up automatically. Instead, I set up two aliases in by .bashrc:

    alias startpostgres='/usr/local/pgsql/bin/pgctl -D /usr/local/pgsql/data -l /usr/local/pgsql/data/logfile start' alias stoppostgres='/usr/local/pgsql/bin/pgctl -D /usr/local/pgsql/data -l /usr/local/pgsql/data/logfile stop'

  • when you start postgres, you'll probably get the dreaded "shared memory error," which looks like this:

    FATAL: could not create shared memory segment: Cannot allocate memory DETAIL: Failed system call was shmget(key=1, size=1081344, 03600). HINT: This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory or swap space. To reduce the request size (currently 1081344 bytes), reduce PostgreSQL's sharedbuffers parameter (currently 50) and/or its maxconnections parameter (currently 10).

  • some resources will tell you to edit the /etc/rc file, which is incorrect for Leopard. Instead, create /etc/sysctl.conf (if it does not already exist -- mine didn't), and add the following lines:

    kern.sysv.shmmax=167772160 kern.sysv.shmmin=1 kern.sysv.shmmni=32 kern.sysv.shmseg=8 kern.sysv.shmall=65536

You'll need to su to root to create /etc/rc

Have fun with Postgres!

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Geocoding Advanced Rails Recipes

Jan 13 by Andre

I have a geocoding recipe coming out in Pragmatic's Advanced Rails Recipes. The recipe is titled "Finding Things by Location," and it offers how-tos and best practices on using my GeoKit plugin to, well, Find Things by Location. The most recent PDF of the beta book includes my recipe along with over 30 other recipes by community leaders like Chris Wanstrath, Dan Benjamin, and Geoffrey Grosenbach, to name just a few.

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Use models to instantiate your Rails fixtures

Jun 27 by Andre
Following is a handy Rake script to import your fixtures (or a subset of your fixtures) through the associated model's constructor.

Say what? You import your fixtures with rake db:fixtures:load FIXTURES=neighborhoods,shops right? Yes, but there's a crucial difference:
  • The standard rake fixtures:load does a straight data load into the database -- your Rails Models are not involved. It's as if you did a series of INSERT INTOs through your MySQL command line.
  • The instantiation script takes the fixture data, and invokes the associated model's constructor for each item.
Which is better? It depends on what you need. I often have logic in my models that is kicked off on instantiation -- in this case, the the instantiation script is crucial, because the model will be forced to to whatever the model is meant to do upon instantiation.

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Transferring a MySQL database structure and data to another machine

May 13 by Andre
This tutorial shows how to transfer a MySQL database from one box to another. There are lots of reasons you might want to do this --my motivation is that I develop on both a desktop at home, and on a laptop which I sometimes want to work on in a disconnected state (i.e., a cafe somewhere). These steps transfer both table structure and data, including keeping all your primary keys intact.

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Rails, Geocoding, and Google Maps

Apr 18 by Andre
Update April '07: GeoKit, my Rails Geocoding plugin, abstracts away all the geocoding logic for multiple providers (Google, Yahoo Geocoder.us, Geocoder.ca -- including failover!), distance-based finders for ActiveRecord, and much more!
This is a simple example to demonstrate how to display a Google Map using Ruby on Rails, including utilizing a geocoding web service to translate addresses to geocodes.

Why the lookup web service? The Google Maps API doesn't just let you map an address. You need to supply the Maps API with a longitude and latitude to place a marker on a map (or even to center a map on a city). The geocoding web service translates a valid address into longitude and latitude; you can pass the result to the Maps API.

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