Group by time span in rails - sql

Question

I want get output from users table based on time of creation of record. Time is stored in created_at column.
Output will be like this:
Time user count
2 am - 6 am 10
6 am - 10 am 5
10 am - 2 pm 5
2 pm - 6 pm 5
6 pm - 10 pm 5
10 pm - 2 am 5
I can't do group by created_at. Solution I found is to create another column say time_span and update that column to 2 am - 6 am if created_at time falls in this span and then I can do group_by on time_span column. Any better solution?

Answer 1

My suggestions is to create another column on the database, this way you avoid calculations on selects at an expense of a simple column.

Answer 2

I'm not sure what you mean by not being able to use group_by, but the following will work:
hours = Users.all.collect {|u| u.created_at.hour}
ranges = [(2...6), (6...10), (10...14), (14...18), (18...22), (22...26)]
summary = hours.group_by {|h| ranges.find {|r| r === (h<2 ? h+24 : h)}}
ranges.each {|r| puts "#{r} = #{(summary[r] || []).length}"}
There are probably opportunities to simplify this and you could push the grouping up into the database if you'd like, but I thought I'd go ahead and share.