ObjectMapper: Find geo places within a certain square, sorted by proximity

I am building a Ruby app on Heroku using Sinatra and a PostgreSQL database interfaced with ObjectMapper. I need to run a query which returns a list of all locations in a database (which each have latitude and longitude attributes) within a certain rectangle (corresponding to the visible map region).

I can do this by searching for latitudes which fall within the map bounds, same for longitude. My question however is, how do I return these results sorted by proximity? I could get all results matching the query and then sort them once they are out of the database, but I want to run this query in batches and return only say the nearest 5 places, then places 6-10, then 11-15, etc.

Can this be done?

EDIT: I have not decided yet wh开发者_运维问答ether to use PostgreSQL for sure, I might use MongoDB if it is appropriate.

The immediate question is proximity to what? You need to define a point to use as the basis for the proximity. You can then use st_distance from the ORDER BY clause to sort by distance between the geometry objects. This can be combined with LIMIT and OFFSET to do exactly what you want.