Question

I need to run a group_by query in Ruby on Rails, but I first want to adjust all records in the created_at column by a certain hour amount before running the query. So, for example, adding 9 hours to every record in the created_at field, and then grouping by date.

Something like the following (which is incorrect):

 @foo = Bar.group("date(created_at + 9.hours)").count

How can I accomplish this in Rails?

Answer 1

PostgreSQL has excellent support for manipulating dates and times (see Date/Time Functions and Operators). You can express ‘9 hours’ as an interval, add it to a timestamp, and cast to a date:

=> select (now()::timestamp + '9 hours'::interval)::date;
    date    
------------
 2012-09-22
(1 row)

This ends up strikingly similar to your original pseudocode:

@foo = Bar.group("date(created_at + '9 hours'::interval)").count