Earliest date after current date without associated record - ruby-on-rails

Question

I have a Rails model DailyAssignment with a date column, and would like to find the first date after today which does not have a DailyAssignment associated with it.
For instance, if I have an instance today, no instance tomorrow, and an instance the day after tomorrow, this method should return tomorrow.
If I were to do this in Ruby, it would be something like:
(Date.today..1.year.since.to_date).find do |date|
DailyAssignment.where(date: date).empty?
end
This is medium okay since it will terminate the iteration once it finds a record, but has two issues:
Iterating through a collection in Ruby is slow.
Barring some sort of while construct, I need to specify an 'end' date.
Is there a nice, efficient way to do this in PostgreSQL?

Answer 1

If you can, you should use a custom query to search through your database (these kind of searches are a lot faster within the DB).
If you search for a date within a time range, you can use the
generate_series(timestamp, timestamp, interval) function:
select s
from generate_series(?, ? + interval '1 year'), interval '1 day') s
left join daily_assignment on s = "date"
where "date" is null
limit 1
If you have no real upper bound, you can use a self-join to get the next free date:
select coalesce(
(select c."date" + interval '1 day'
from daily_assignment c
left join daily_assignment n on n."date" = c."date" + interval '1 day'
where c."date" > ? - interval '1 day'
and n."date" is null
order by c."date"
limit 1),
? + interval '1 day'
)
? marks mean the parameter of today (you may need casts, depending on your input); you could use now() instead, if you prefer.
P.S.: please, do not use date as a column name, it is a reserved word in SQL, and tells nothing about the column itself. Instead, you can use names like created_at, updated_at, happens_at, etc. or even at_date.

Answer 2

What I propose is to do 1 select query between dates, then loop your results and compare them with your selected results.
# select all dailyassignments
results = DailyAssignment.where("date >= from_date AND date <= to_date")
not_found_dates = []
(Date.today..1.year.since).find do |date|
found_assignment = results.detect {|instance| instance.date == date }
not_found_dates << date if found_assignment.nil?
end

Answer 3

You can try it this way:
def first_date_without_assignment
assignments = DailyAssignment.select('date').where('date > ?', Date.today)
return Date.tomorrow if assignments.empty?
assignment_dates = assignments.map(&:date)
date_range = (Date.tomorrow..(assignment_dates.last.advance(days: 1)).to_a
(date_range - assignment_dates).first
end
I didn't test it so I could mistype something, but it could work. I also find this, it should work on postgres http://www.postgresql.org/message-id/4F96EC90.6070600#encs.concordia.ca but it could be quite hard to write in rails or at least bad looking.