Select oldest HABTM record with group by clause - sql

Question

I want to show a line chart on the admin page (with chartkick) with the incremental number of scores related to their earliest export date.
I have the following models:
# score.rb
class Score < ApplicationRecord
has_and_belongs_to_many :export_orders, join_table: :scores_export_orders
end
# export_order.rb
class ExportOrder < ApplicationRecord
has_and_belongs_to_many :scores, join_table: :scores_export_orders
end
How do I select, for each Score having at least one ExportOrder, the corresponding ExportOrder with the earliest created_at (in date only format)?
I had a look at this, but my situation has a HABTM relationship instead of a simple has_many.
I tried this code, to get at least a mapping between oldest export date and number of scores:
sql = "
SELECT
COUNT(DISTINCT scores.id), MIN(export_orders.created_at::date)
FROM
scores
INNER JOIN
scores_export_orders
ON
scores.id = scores_export_orders.score_id
INNER JOIN
export_orders
ON
export_orders.id = scores_export_orders.export_order_id
GROUP BY
export_orders.created_at::date
".split("\n").join(' ')
query = ActiveRecord::Base.connection.execute(sql)
query.map { |v| [v['count'], v['min']] }
but the total number of scores is greater than all scores having an export date.
Any ideas?

Answer 1

Try:
class Score < ApplicationRecord
has_and_belongs_to_many :export_orders, join_table: :scores_export_orders
def earliest_export_date
export_orders.pluck(&:created_at).min
end
end
This will let you call #score.earliest_export_date, which should return the value you want.
I also think it's the most performant way to do it in ruby, although someone may correct me on that.

Answer 2

The following has better performance than Mark's solution since it relies on pure SQL. Basically, the GROUP BY clause required grouping by scores_export_orders.score_id rather than export_orders.created_at:
sql = "
SELECT
COUNT(DISTINCT scores_export_orders.score_id), MIN(export_orders.created_at::date)
INNER JOIN
scores_export_orders
INNER JOIN
export_orders
ON
export_orders.id = scores_export_orders.export_order_id
GROUP BY
scores_export_orders.score_id
".split("\n").join(' ')
query = ActiveRecord::Base.connection.execute(sql)
query.map { |v| [v['count'], v['min']] }
I couldn't find an exact equivalent in ActiveRecord instructions (all of such attempts were giving me strange results), so executing the SQL will also do the trick.