Ruby/SQLite code for finding places within a certain distance from another place. In other words, Ruby code that adds a "DISTANCE(lat1, lon1, lat2, lon2)" to SQLite, allowing you to select records based on the distance between two points of latitude and longitude. Magic! Adapted from the ObjectiveC version here: http://daveaddey.com/?p=71

sqlite-distance.rb

db = SQLite3::Database.new ":memory:"

# A sample SQLite table; all that's necessary is the lat and log fields
db.execute <<-SQL
CREATE TABLE users (
  email VARCHAR(255),
  lat FLOAT,
  lon FLOAT
)
SQL

# Insert/load your records here, if you like. Personally, I'm loading a CSV file.

def deg2rad(degrees)
  # radians = degrees * (pi / 180)
  degrees * 0.01745327
end

db.create_function('distance', 4) do |func, lat1, lon1, lat2, lon2|
  lat1rad = deg2rad(lat1)
  lat2rad = deg2rad(lat2)
  func.result = acos(sin(lat1rad) * sin(lat2rad) + cos(lat1rad) * cos(lat2rad) * cos(deg2rad(lon2) - deg2rad(lon1))) * 6378.1
end

# That's it. You can now find records within a certain distance:
users_near_chelsea = db.execute('SELECT COUNT(*) FROM users WHERE DISTANCE(lat, lon, 51.48282, -0.17406) <= 80')

# (In that case, records where the location is within 80km of a randomly selected point near our office.)