Annoying find thingy

I want to know which thingy is centered on a given city. Both thingy
and city have a lat and a lng attribute, ad lat and lng are both
floats in the mysql database.

knowing how clever ActiveRecord::Base is I wanted to do:

City.find_by_lat_and_lng(self.lat, self.lng)

but that interprets lat and lng as

SELECT * FROM cities WHERE (cities.lat = ‘50.12’ AND
cities.lng = ‘-5.7’) LIMIT 1

and since lat/lng are floats that won’t work,

I would have thought that

City.find(:first, :conditions => [ “lat = ? AND lng = ?”, self.lat,
self.lng ])

SELECT * FROM cities WHERE (lat = 50.13 AND lng = -5.56) LIMIT 1

would work but it returns nothing.

what I had to do was

City.find(:first, :conditions => [ “lat LIKE ? AND lng LIKE ?”,
self.lat, self.lng ])

which becomes

SELECT * FROM cities WHERE (lat LIKE 50.13 AND lng LIKE -5.56)
LIMIT 1

and works.

Surely the first case is an ActiveRecord::Base problem in that it
ought to interpret a float as a float and not a string (LIKE), the
second is a mysql problem in that the query is good SQL and the third
is just wrong, although it does work.

float just does not play well with ‘==’, however represented. You
nearly always need to provide some form of tolerance to allow for the
inherent imprecision of
the internal representation. Given the application, you probably need
to provide some circular probability of error regarding fixing the
spherical co-ordinate centre in any case.

Col,

Take a peek at GeoKit: http://geokit.rubyforge.org/

BTW, use Decimal (BigDecimal?) for your lats & lngs. Better precision.
To wit, I have an Addresses table declared as follows:

create_table :addresses, :force => true do |t|

t.string :full_address, :street_address, :city, :state, :zip, :country
t.decimal :lat, :lng, :precision => 15, :scale => 10
end

Works great for me. GeoKit lets you do all the lookup goodness,
including some nice “find” overrides.

-Danimal

On Apr 11, 1:33 pm, Col W. [email protected]

Not sure if it applied to what you’re doing, but it seems like you’d
have a tolerance issue even without the imprecision of the float. My
city is roughly 40N 86W, but that wouldn’t find it in an ‘=’ query.
Maybe a between query with a fudge factor, like:

:conditions => [“lat between ? and ? and lon between ? and ?”,lat-
fudge,lat+fudge,lon-fudge,lon+fudge]

To keep it DRY, you could build your own finder method on city.

-Mack

On Apr 11, 8:09 pm, Col W. [email protected]

Thanks for that.

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