1

I'm working on Rails application that let users to make max 2 posts for each day, very simple.

My problem is how to manage different timezone where different users lives. For example if I live in London and the day start at 00.00 and finish at 23.59 the posts count will reset at 00.00, but for another users live in New York the counter will then reset at different time. (not at 00.00) How can I manage this situation?

I hope I explained myself.

UPDATE 1

@Cluster

The problem with your code is that time_zone.utc_offset give a wrong time delta:

1.9.3p194 :123 > time_zone = ActiveSupport::TimeZone.new("Prague")
 => (GMT+01:00) Prague 
1.9.3p194 :124 > DateTime.now.utc.midnight - time_zone.utc_offset
 => Thu, 10 Apr 2003 00:00:00 +0000

Why that?

Instead of that I've found useful this code:

User.posts.where(:created_at => DateTime.now.in_time_zone(time_zone).beginning_of_day..DateTime.now.in_time_zone(time_zone).end_of_day).count

It seems to get the correct number of messages between the user's day (with user's timezone). What do you think?

UPDATE 2

@Cluster

What's the difference between:

User.posts.where(:created_at => DateTime.now.in_time_zone(utc_time_zone).beginning_of_day..DateTime.now.in_time_zone(utc_time_zone).end_of_day).count

and yours:

1.9.3p327 :015 > time_zone = ActiveSupport::TimeZone.new("Prague")
 => (GMT+01:00) Prague 
1.9.3p327 :016 > Time.zone.now.utc.midnight - time_zone.utc_offset
 => 2013-02-15 23:00:00 UTC

in terms of performances and way of doing?

UPDATE 3

Actually the code in your example doesn't work, look here:

# WRONG
1.9.3p194 :096 > user.posts.where('created_at > ?', Time.zone.now.utc.midnight - my_time_zone.utc_offset).count
   (0.2ms)  SELECT COUNT(*) FROM "messages" WHERE "messages"."sender_id" = 5 AND (created_at > '2013-02-15 23:00:00.000000')
 => 4 

# CORRECT
1.9.3p194 :098 > users.posts.where(:created_at => DateTime.now.in_time_zone(my_time_zone).beginning_of_day..DateTime.now.in_time_zone(my_time_zone).end_of_day).count
   (0.3ms)  SELECT COUNT(*) FROM "messages" WHERE "messages"."sender_id" = 5 AND ("messages"."created_at" BETWEEN '2013-02-16 23:00:00.000000' AND '2013-02-17 22:59:59.000000')
 => 0
Fred Collins
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    In response to Update 1, it is because DateTime is subtracting Days instead of Seconds, should have known better, this is why I always use Time.zone.now instead of DateTime.now. Time.zone is a rails addition that gives you a `ActiveSupport::TimeWithZone` which seems to be saner than Date/Time/DateTime, changed my response to reflect that – Cluster Feb 16 '13 at 21:56

2 Answers2

4

Depending on exactly how you want it to work I can think of two solutions.

First, allowing the user to post twice in a 24 hour period, this has the benefit of being timezone agnostic:

User.posts.where('created_at > ?', 24.hours.ago).count

If that returns 2 then don't let them post.

If you really want to ensure that there are no more than 2 posts per day then:

time_zone = ActiveSupport::TimeZone.new(current_user.time_zone)
Users.posts.where('created_at > ?', Time.zone.now.utc.midnight - time_zone.utc_offset).count

User#time_zone needs to hold a valid time zone recognized by Rails.

Since the database datetime (created_at) is already in UTC, first normalize your server time to utc, then set the time to midnight, and apply the utc offset for the timezone. So if you have someone in a -7 timezone, then it will look for posts since 0700 UTC, for someone in a +3 timezone it will look for posts since 2100 UTC.

ActiveSupport::TimeZone

To clarify the difference between the two let me use a couple of examples.

First example is a user posting at 1200 then again at 1800 on Monday.

With the first piece of code, the user will not be able to post again until after 0600 Tuesday. With the second piece of code they will be able to post again at 0000 Tuesday (after midnight).

Second example is a user posts at 2330 then again at 2350 on Monday. Again, in the first example they will not be able to post again until after 2330 Tuesday, but the second example will allow them to post again at midnight, allowing them to post at 0010 and 0020, giving them 4 posts in 50 minutes.

It really depends on what the purpose of the post limitation is for. The first example is going to be much simpler to use and implement as it will work irregardless of time zones.

If your pulling time zone info from FB, then check what format that timezone info is in. I'm guessing it will not be a valid string that ActiveSupport::TimeZone will accept. For example it wants "Mountain Time (US & Canada)" for MST(-0700). If FB is giving you back something like -7 for MST or -25200 (-7 hours in seconds) then you can use those without mucking around with AS:TZ

RE: Update 1

Here's the output using Time.zone instead of DateTime

1.9.3p327 :015 > time_zone = ActiveSupport::TimeZone.new("Prague")
 => (GMT+01:00) Prague 
1.9.3p327 :016 > Time.zone.now.utc.midnight - time_zone.utc_offset
 => 2013-02-15 23:00:00 UTC
Cluster
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  • Thanks for the answer man! What you mean when you say: "If you really want to ensure that there are no more than 2 posts per day then.."? The first method with `24.hours.ago` is not good? And for the second method I need a `time_zone` field in the `User` model, right? I let the users connect only with FB, so I need to take the time zone from FB, I think it's possible without any troubles. Btw I would manage this situation in this way: if you have already posted 2 post today (between YOUR timezone 00.00-23.59) and passes midnight (so it's the new day) you must be able to make your new 2 posts. – Fred Collins Feb 16 '13 at 20:11
  • Updated with an explanation clarifying the differences between the two examples. – Cluster Feb 16 '13 at 21:50
  • Good answer dude, very thanks. Please check out my new update. – Fred Collins Feb 16 '13 at 23:34
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Consider to use UTC timezone for everyone. I.e. StackOverflow uses that approach for votes and it works fine.

Alexei Levenkov
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  • Thanks. How can I use UTC timezone for achieve this? Did you have some question/answer or link that provides some useful information for Rails? – Fred Collins Feb 16 '13 at 19:21
  • No... Actually what you are trying to achieve? - let user post once in 24 hours (than just use your server's timezone) OR post twice during user 0:00-23.59 period (than you need to send user's timezone from browser [getTimeZoneOffset](https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Date/getTimezoneOffset) and send it to your server). – Alexei Levenkov Feb 16 '13 at 19:31
  • I would let user post once in his day (00:00 - 23.59 period), not in server's timezone). For example if you live in a different timezone from me, your day may finish before or after mine, and then your posts counter may reset at YOURS end of the day, which is different from mine. – Fred Collins Feb 16 '13 at 19:40
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    @FredCollins, note that doing "user's day" correctly is harder that server's/UTC timezone - you need to take care of daylight savings, user crossing timezone in a day. See if the difference actually is important to you... Anyway Cluster (+1) gave you code, so the only piece you still need is search for [ruby user timezone rails](http://stackoverflow.com/questions/12326570/rails-how-to-get-the-current-users-time-zone-when-using-heroku) – Alexei Levenkov Feb 16 '13 at 20:42