How to create a finder condition that operates on the associated model - ruby-on-rails

Question

Assuming rails app has the following models song, play, radio_station:
song has_many plays
play has_many radio_stations
radio_station has attribute "call_sign"
I want to create one find for Song that would return for me the all songs where the last play (of the song) was played by a radio station of "WKRP"
I could do
found = []
Song.find(:all,:include=>[{:plays=>[:radio_station]}]).each do |song|
found << song if song.plays.last.radio_station.call_sign == "WKRP"
end
but this would mean that all songs must be pulled down from the DB first and looped through... which would get slow as the # of songs and plays builds.
How can I put this into one find condition?
It seems like this is something that should be doable - but I can't figure out how and I am not really an SQL guru ...
and then.. would love to squeeze it into a named scope so i could call it like:
Song.find_all_last_played_at("WKRP")

Answer 1

This probably can't be done by ActiveRecord finders. You have to roll some custom SQL with a subquery (MySQL 5 required):
SELECT s.* FROM songs s LEFT JOIN plays p
WHERE p.radio_station_id = [id of WKRP]
AND p.created_at = (SELECT MAX(created_at) FROM plays p2
WHERE p2.song_id = s.id)
Load these songs by using find_by_sql:
Song.find_by_sql('SELECT ...')
You could refactor it using a named_scope if you want to.
class Song < ActiveRecord::Base
has_many :plays
named_scope :last_played_on, lambda { |radio_station|
{ :conditions => ['plays.created_at = (SELECT MAX(p2.created_at) FROM plays p2
WHERE p2.song_id = songs.id) AND plays.radio_station_id = ?', radio_station.id],
:joins => [:plays] }
}
end
#wkrp = RadioStation.find_by_name('WKRP')
Song.last_played_on(#wkrp).each { |song| ... }
Note that using subqueries can be slow. You may want to cache the latest play_id of a song as a field in the songs table to make the queries easier and faster. Start by adding a last_play_id field to your songs table.
class Play < ActiveRecord::Base
belongs_to :song
after_create do |play|
play.song.last_play_id = play.id
play.song.save!
end
end

Answer 2

You should probably look at named scopes.
class Song
named_scope :last_played_by, lambda {|call_sign| {:conditions => ["plays.radio_stations.call_sign = ?", call_sign]}}
end
You would use it as such
Song.last_played_by("WKRZ")
However, I realized that I don't know what mechanism you are using to find the last play. But you could split it up into two named scopes as such:
class Song
named_scope :played_by, lambda {|call_sign| {:conditions => ["plays.radio_station.call_sign = ?", call_sign]}}
named_scope :last_play {:conditions => ["MAX(plays.created_at)"]
end
To use it:
Song.last_play.played_by("WKRZ")
I believe that is correct, but I haven't tested it out and I'm not all that familiar with named scopes.

Answer 3

For performance, you should probably do what wvangergen suggests. But for syntatic sugar, I think this might work:
class Song
has_many :plays, :order => 'created_at' do
def last_played
self.maximum(:created-at)
end
end
end
class Play
named_scope :by, lambda {|call_sign| {:conditions => ["radio_stations.call_sign = ?", call_sign]}}
end
To use:
Song.plays.last_played.by("WKRZ")
I'm not testing the code out, so apologies for the syntax errors =) Now that I'm reading it, I don't think it will work, but I hope you might be able to figure it out or something.

Answer 4

I think you can do that starting with your associations (Note that the default 'order' is 'created_at'):
class Song
has_many :plays
has_many :radio_staions, :through => :plays, :uniq => true
has_one :last_radio_station, :through => :plays, :source => :last_radio_station
named_scope :last_played_at, lambda {|call_sign| {:include => :last_radio_station, :conditions => ["radio_stations.call_sign = ?", call_sign]}}
end
This expects your Play model to have:
class Play
has_one :last_radio_station, :class_name => 'RadioStation', :order => 'created_at'
has_many :radio_stations
Then you can simply call:
Song.last_played_at("WKRZ")
I haven't tried this myself, but hopefully it will work.

Answer 5

Created a named_scope that pulls all the plays for that particular radio station, with order 'created_at DESC' and then chain .last on to the end of the returned query. Along the lines of:
Song.played_by('WRXD').last