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= weighted_average
Do weighted averages in ActiveRecord.
== Rationale
You have a bunch of flight records with passenger count and distance.
* Flight EWR <-> MSN; 30,000 passengers last month; 500 miles
* Flight EWR <-> BOM; 15 passengers last month; 10,000 miles
The average distance is <tt>(10_000 + 500) / 2 = 5250</tt>.
The average distance weighted by passenger count is <tt>(30_000 * 500 + 15 * 10_000) / (10_500) = 1442</tt>.
== Usage
Using <tt>FlightSegment</tt> from {Brighter Planet's}[http://brighterplanet.com] {earth gem}[http://rubygems.org/gems/earth]:
>> FlightSegment.weighted_average(:distance, :weighted_by => :passengers)
=> 2436.1959
You can also see the SQL that is generated:
>> FlightSegment.weighted_average_relation(:distance, :weighted_by => :passengers).to_sql
=> "SELECT (SUM((`flight_segments`.`distance`) * `flight_segments`.`passengers`) / SUM(`flight_segments`.`passengers`)) AS weighted_average FROM `flight_segments` WHERE (`flight_segments`.`distance` IS NOT NULL)"
== Copyright
Copyright (c) 2010 Seamus Abshere, Andy Rossmeissl, Ian Hough, and Matt Kling. See LICENSE for details.
Version data entries
7 entries across 7 versions & 1 rubygems