75 lines
2.3 KiB
Markdown
75 lines
2.3 KiB
Markdown
---
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title: "Protip: TimeWithZone, All The Time"
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date: 2008-09-10T00:00:00+00:00
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draft: false
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canonical_url: https://www.viget.com/articles/protip-timewithzone-all-the-time/
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---
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If you've ever tried to retrieve a list of ActiveRecord objects based on
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their timestamps, you've probably been bitten by the quirky time support
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in Rails:
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```
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>> Goal.create(:description => "Run a mile")
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=> #<Goal id: 1, description: "Run a mile", created_at: "2008-09-09 19:32:57", updated_at: "2008-09-09 19:32:57">
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>> Goal.find(:all, :conditions => ['created_at < ?', Time.now])
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=> []
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````
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Huh? Checking the logs, we see that the two commands above correspond to
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the following queries:
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```sql
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INSERT INTO "goals" ("updated_at", "description", "created_at") VALUES('2008-09-09 19:32:57', 'Run a mile', '2008-09-09 19:32:57')
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SELECT * FROM "goals" WHERE created_at < '2008-09-09 15:33:17'
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````
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Rails stores `created_at` relative to [Coordinated Universal
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Time](https://en.wikipedia.org/wiki/Coordinated_Universal_Time), while
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`Time.now` is based on the system clock, running four hours behind. The
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solution? ActiveSupport's
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[TimeWithZone](http://caboo.se/doc/classes/ActiveSupport/TimeWithZone.html):
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```
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>> Goal.find(:all, :conditions => ['created_at < ?', Time.zone.now])
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=> [#<Goal id: 1, description: "Run a mile", created_at: "2008-09-09 19:32:57", updated_at: "2008-09-09 19:32:57">]
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```
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**Rule of thumb:** always use TimeWithZone in your Rails projects. Date,
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Time and DateTime simply don't play well with ActiveRecord. Instantiate
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it with `Time.zone.now` and `Time.zone.local`. To discard the time
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element, use `beginning_of_day`.
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## BONUS TIP
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Since it's a subclass of Time, interpolating a range of TimeWithZone
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objects fills in every second between the two times --- not so useful if
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you need a date for every day in a month:
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```
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>> t = Time.zone.now
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=> Tue, 09 Sep 2008 14:26:45 EDT -04:00
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>> (t..(t + 1.month)).to_a.size [9 minutes later]
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=> 2592001
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```
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Fortunately, the desired behavior is just a monkeypatch away:
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```ruby
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class ActiveSupport::TimeWithZone
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def succ
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self + 1.day
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end
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end
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>> (t..(t + 1.month)).to_a.size
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=> 31
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```
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For more information about time zones in Rails, [Geoff
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Buesing](http://mad.ly/2008/04/09/rails-21-time-zone-support-an-overview/)
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and [Ryan
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Daigle](http://ryandaigle.com/articles/2008/1/25/what-s-new-in-edge-rails-easier-timezones)
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have good, up-to-date posts.
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