Rails - Nearby cities in existing query which joins city table - ruby-on-rails-3

Question

I have the following models:
class User
has_one :player
belongs_to :city
end
class Player
belongs_to :user
end
class City
has_many :users
end
Each City has a latitude and longitude attributes.
I have the following query (it's a little bit longer, but what matters is the city part) that I use to get players which associated users are from a particular city. As this:
def self.search(city)
self.includes(:user => :city).
where(['cities.name = ?', city])
end
How would I change this query to get professors which users associated cities are near by (10 kilometers for example) the given city?
EDIT: I have this method in the City model which gives me the closest city given a particular radius (in km), latitude and longitude:
def near_by(latitude,longitude,radius)
City.select("cities.*,
6371 *
acos(cos(radians(#{latitude})) *
cos(radians(cities.latitude)) *
cos(radians(cities.longitude) - radians(#{longitude})) +
sin(radians(#{latitude}))*sin(radians(cities.latitude)))
AS km_away").group('km_away ASC').having('km_away <= ?', radius)
end
EDIT2: Here is what I have so far in order to combine both queries:
# Pass a city object to the search instead of a city name, so we can use lat and long
def self.search(city)
self.includes(:user => :city).
where(['cities.id IN (?)', City.near_by(city.latitude,city.longitude)])
end
But is this a good approach? If so, how can I get the professors ordered by proximity to the passed city???

Answer 1

For working with geographic data, I recommend you check out the Geocoder gem as described in this railscast.
From the readme:
With geocoded objects you can do things like this:
obj.nearbys(30) # other objects within 30 miles
obj.distance_from([40.714,-100.234]) # distance from arbitrary point to object
obj.bearing_to("Paris, France") # direction from object to arbitrary point