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Geocoder adds object geocoding and database-agnostic distance calculations to Ruby on Rails. It's as simple as calling <tt>fetch_coordinates!</tt> on your objects, and then using a scope like <tt>Venue.near("Billings, MT")</tt>. Since it does not rely on proprietary database functions finding geocoded objects in a given area works with out-of-the-box MySQL or even SQLite.
Geocoder is compatible with Rails 2.x and 3.x. <b>This is the README for the 3.x branch.</b> Please see the 2.x branch for installation instructions, documentation, and issues.
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gem "rails-geocoder", :require => "geocoder"
rails plugin install git://github.com/alexreisner/geocoder.git
A) Add +latitude+ and +longitude+ columns to your model:
rails generate migration AddLatitudeAndLongitudeToYourModel latitude:float longitude:float
B) Tell geocoder where your model stores its address:
C) Optionally, auto-fetch coordinates every time your model is saved:
after_validation :fetch_coordinates
<i>Note that you are not stuck with the +latitude+ and +longitude+ column names, or the +address+ method. See "More On Configuration" below for details.</i>
Assuming +obj+ is an instance of a geocoded class, you can get its coordinates:
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obj.fetch_coordinates # fetches and assigns coordinates
obj.fetch_coordinates! # also saves lat, lon attributes
If you have a lot of objects you can use this Rake task to geocode them all:
rake geocode:all CLASS=YourModel
obj.nearbys(30) # other objects within 30 miles
obj.distance_to(40.714, -100.234) # distance to arbitrary point
To find objects by location, use the following scopes:
Venue.near('Omaha, NE, US', 20) # venues within 20 miles of Omaha
Venue.near([40.71, 100.23], 20) # venues within 20 miles of a point
Venue.geocoded # venues with coordinates
Venue.not_geocoded # venues without coordinates
Some utility methods are also available:
# distance (in miles) between Eiffel Tower and Empire State Building
Geocoder.distance_between( 48.858205,2.294359, 40.748433,-73.985655 )
# look up coordinates of some location (like searching Google Maps)
Geocoder.fetch_coordinates("25 Main St, Cooperstown, NY")
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# find the geographic center (aka center of gravity) of objects or points
Geocoder.geographic_center([ city1, city2, city3, [40.22,-73.99], city4 ])
== More On Configuration
You are not stuck with using the +latitude+ and +longitude+ database column names for storing coordinates. For example, to use +lat+ and +lon+:
geocoded_by :address, :latitude => :lat, :longitude => :lon
The string to use for geocoding can be anything you'd use to search Google Maps. For example, any of the following are acceptable:
714 Green St, Big Town, MO
Eiffel Tower, Paris, FR
Paris, TX, US
If your model has +address+, +city+, +state+, and +country+ attributes you might do something like this:
geocoded_by :location
def location
[address, city, state, country].compact.join(', ')
end
Please see the code for more methods and detailed information about arguments (eg, working with kilometers).
SQLite's lack of trigonometric functions requires an alternate implementation of the +near+ method (scope). When using SQLite, Geocoder will automatically use a less accurate algorithm for finding objects near a given point. Results of this algorithm should not be trusted too much as it will return objects that are outside the given radius.
=== Discussion
There are few options for finding objects near a given point in SQLite without installing extensions:
1. Use a square instead of a circle for finding nearby points. For example, if you want to find points near 40.71, 100.23, search for objects with latitude between 39.71 and 41.71 and longitude between 99.23 and 101.23. One degree of latitude or longitude is at most 69 miles so divide your radius (in miles) by 69.0 to get the amount to add and subtract from your center coordinates to get the upper and lower bounds. The results will not be very accurate (you'll get points outside the desired radius--at worst 29% farther away), but you will get all the points within the required radius.
2. Load all objects into memory and compute distances between them using the <tt>Geocoder.distance_between</tt> method. This will produce accurate results but will be very slow (and use a lot of memory) if you have a lot of objects in your database.
3. If you have a large number of objects (so you can't use approach #2) and you need accurate results (better than approach #1 will give), you can use a combination of the two. Get all the objects within a square around your center point, and then eliminate the ones that are too far away using <tt>Geocoder.distance_between</tt>.
Because Geocoder needs to provide this functionality as a scope, we must go with option #1, but feel free to implement #2 or #3 if you need more accuracy.
== Known Issue
You cannot use the +near+ scope with another scope that provides an +includes+ option because the +SELECT+ clause generated by +near+ will overwrite it (or vice versa). Instead, try using +joins+ and pass a <tt>:select</tt> option to the +near+ scope to get the columns you want. For example, in Rails 2 syntax:
City.near("Omaha, NE", 20, :select => "venues.*").all(:joins => :venues)
If anyone has a more elegant solution to this problem I am very interested in seeing it.
* use completely separate "drivers" for different AR adapters?
* seems reasonable since we're using very DB-specific features
* also need to make sure 'mysql2' is supported
* make 'near' scope work with AR associations
* http://stackoverflow.com/questions/3266358/geocoder-rails-plugin-near-search-problem-with-activerecord
* prepend table names to column names in SQL distance expression (required
to do joins on another geocoded model)
* unobtrusively add ability to get a result with more data (Geocoder object?)
* the default usage should remain dead simple
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Copyright (c) 2009-10 Alex Reisner, released under the MIT license