Ruby Gem Squeel, how to write self join - sql

Question

I'm trying to write the following query using the ruby gem Squeel
SELECT COUNT(*)
FROM(
SELECT
a.end_at AS START,
Min(b.start_at) AS END
FROM periods AS a
JOIN periods AS b ON b.season_id IN (1,2,3) AND a.end_at <= b.start_at
WHERE a.season_id IN (1,2,3)
GROUP BY a.end_at
HAVING a.end_at < MIN(b.start_at)
) AS gaps
WHERE
gaps.START < '2013-05-17' AND gaps.END > '2013-05-05';
Any idea on how to achieve this?
I can get the seasons_ids using:
seasons = self.seasons
Period.where{season_id.in(seasons.select{id})}
but the self join with condition, I have no idea how to tackle this so far.

Answer 1

This is similar to what I've tried to do here.
What I would try is using a sub-query, joining Period to itself as follows:
class Period < ActiveRecord::Base
attr_accessible :end_at, :season_id, :start_at
belongs_to :season
scope :in_seasons, ->(season_ids) { joins{season}.where{id.in(season_ids)} }
def self.find_gaps(start_date, end_date)
season_ids = ["1", "2", "3"]
scope = select{[end_at.as(`gap_start`), `min(self_join.start_at)`.as(`gap_end`)]}
scope = scope.joins{"LEFT JOIN (" + Period.in_seasons(season_ids).to_sql + ") AS self_join ON self_join.end_at <= periods.start_at"}
scope = scope.where{(`self_join.start_at` != nil) & (`gap_start` < start_date) & (`gap_end` > end_date)}
scope = scope.group{`gap_end`}.having{end_at < `min(self_join.start_at)`}
end
end
In rails console, Period.find_gaps('2001-01-01', '2010-01-01').to_sql produces (formatting is my own):
SELECT
\"periods\".\"end_at\" AS gap_start,
min(self_join.start_at) AS gap_end
FROM \"periods\"
LEFT JOIN
(SELECT \"periods\".*
FROM \"periods\"
INNER JOIN \"seasons\" ON \"seasons\".\"id\" = \"periods\".\"season_id\"
WHERE \"periods\".\"id\" IN (1, 2, 3)
) AS self_join ON self_join.end_at <= periods.start_at
WHERE ((self_join.start_at IS NOT NULL AND gap_start < '2001-01-01' AND gap_end > '2010-01-01'))
GROUP BY gap_end
HAVING \"periods\".\"end_at\" < min(self_join.start_at)
It looks like what you are trying to get at... at least the inner portion.
Hope it helps.