ruby - Rails: Query database for users with specific time value while respect individual users time zone setting -

i have background job runs every 15 minutes , generates reminder emails. create query returns users have specific time saved , respect how timezone setting effects time.

so have user model stores:

• :time: users reminder time, eg 17:00:00
• :string: timezone, eg est

so if job runs @ 17:00:00 est, return users settings are:

• reminder_time: 17:00:00, time_zone: est
• reminder_time: 13:00:00, time_zone: pst

what best way build query? can done in 1 pass, relying on postgres handle work? have stagger it, group each time zone , doing math each on in ruby?

i have setup activerecord scope doesn't consider timezones, , trying add consideration now.

scope :receives_reminder_at, -> (time)   ready.where(reminder_time: time) end 

when dealing users in multiple timezones, easiest standardize on utc.

so store reminder_time in utc, way don't have worry tz when querying, since normalized utc. (assuming running servers utc. work expected). use tz offset in order adjust time viewing.