Geocoder Gem .near可以工作超过1000

Question

I am using the Geocoder Gem. When I use .near I get no results unless I set the distance to 500 to 1000.

I created this query:

User.near([53.2910642, -6.1969235], 20).any?

But do not get any results, but:

User.near([53.2910642, -6.1969235], 2000).any?

Does. I have a user in the database.

I have a user in the database with a latitude of 53.2910642 and longitude of -6.1969235. These are stored in Float columns and in the User model I have:

#Geocoding
geocoded_by :address
after_validation :geocode

Is their something obvious that I am missing? I am testing this in the test environment, in the Rails Console.

Answer 1

Depending on the database backend you're using, floating-point precision might be different than what you expect.

Eg you might want to check that you're at least using a precision of 15 and a scale of 10. A rails column migration would be created using:

[ :longitude, :latitude ].each do |c|
  t.decimal c, precision: 15, scale: 10
  t.decimal c, precision: 15, scale: 10
end

Also, if you're planning to query your database for entries based on their location, make sure you're using correct index for those columns!

The queries that are issued to a database backend for querying a distance between two coordinates could be heavy. When I worked on similar projects, I ended up rewriting this kind of functionality to allow my models to store intermediate values.

For instance, if your distance computation formula looks like the one described here: http://www.movable-type.co.uk/scripts/latlong.html , you might want to create indexed columns to store the result of φ1 and φ2, as well as columns to store results of the cosine / sine of φ1 and φ2.

This way, you will be able to optimize the database queries and greatly improve run times.

Another approach I ended up using was to do a rough bounding-box based SQL query based on the radius, returning a list of candidates for a more complex geodesic based formula. This way, the geodesic SQL formula could either pre-select candidates using the IN [...] operator, or even completely be by-passed and be done with your code using the candidates array.