ActiveRecord and attributes returned

I am working with a couple of queries where I need to get the latitude and longitude from a table based on the zip code I pass to it.

@geoLatitude = Geodemog.find_by_sql["select latitude from geodemogs where post_code = ?", @query]

I want to:

1) Make sure this is the best way to do this since the table holds a lot of records 2) Am I setting this up properly to be able to send a second query to Mysql where I will use the variable @geoLatitude to do the actual calculations?

Thank you in advance for your help!

-Scott