Closest day in array sql - sql

Question

I'm trying to find a record and order it by the closets day based on the current day.
Let me try to illustrate with an example.
Say that John wants to find out when he has to teach next. John teaches the following days (days are converted to numbers, where 0 = Sunday, 1 = Monday ...) [1,2]. The current day is Friday (5) and so the result should be 1.
Another example:
Karen wants to figure out when she has to teach next. Karen teaches the following days [0,2,3]. The current day is Thursday (4) and so the result should be 0.
Current query:
TeamOverview.where(coach: current_user.id).order(:day = [next closets day missing here])
Model:
t.string :name
t.integer :coach
t.int :day
Possible records in model:
id: 1, name: John, day: 1
id: 2, name: John, day: 2
id: 3, name: Karen, day: 0
id: 4, name: Karen, day: 2
id: 5, name: Karen, day: 3

Answer 1

To avoid a complex solution I'd rather select all working days and do the work in ruby. Max number of results from query is 7 - that should be fine. Additionally changes are high that you already have such a query (maybe scoped on TeamOverview), you can reuse this and on a second call it may also come from query cache.
working_days = TeamOverview.where(coach: current_user.id).pluck(:day)
working_days.find { |day_num| day_num > today_num } || working_days.first

Answer 2

For getting the next day sorted top you will need to order by distance from current day.
Unfortunately, the %(modulo operator with postgresql keeps the sign
for negative values. Thus, you can not use day - current_day.
But the ordering on following expression will sort the rows according to your intened logic:
(7 + day - current_day) % 7
WHere day is the day form the model row and current_dayis the value for the current day to use with the calculation.
If you prefer a more explicit solution you might turn to
CASE WHEN day < current_day THEN 7 + day - current_day ELSE day - current_day END

Answer 3

(res =TeamOverview.where(coach: current_user.id).order("day").find(:first, :conditions => [ "(day > ? )",current_day]) ) ? res : TeamOverview.where(coach: current_user.id).order("day").find(:first, :conditions => [ "(day < ? )",current_day])
This will give you desired result

Answer 4

You can achieve this in pure SQL with a two-part ordering:
SELECT *
FROM team_overviews
ORDER BY day < (current_day), day
A simple implementation in ActiveRecord for this is:
TeamOverview
.where(coach: current_user.id)
.order("day < #{current_day}")
.order(:day)
Or, if you prefer:
TeamOverview
.where(coach: current_user.id)
.order("day < #{current_day}, day")
Warning: The above code is potentially vulnerable to SQL injection. Be sure that you trust the source of current_day, and sanitise (in this case, probably just call to_i) if necessary.
Unfortunately, looking at the rails source code (as of the current 4.2.6 version at time of posting), it looks as though the ? syntax is not supported by the order ActiveRecord method - i.e. the following is invalid: TeamOverview.order("day < ?", current_day). Perhaps that would be a good addition to the library.

Answer 5

Following code will find next upcoming day, if you need days then remove .first argument
next_day = TeamOverview.where("day > ? and coach_id = ?", current_day, current_user.id).order("day").first