Change default_timezone of active record - ruby-on-rails

Question

In rails, I have the below config for activerecord at first.
config.active_record.default_timezone = :utc
Now, I want to use the local timezone, so I changed it to:
config.active_record.default_timezone = :local
The problem is, I need to shift all the existing data in the date/datetime column to the local timezone.
What is the best way to achieve this?
Why I have to do this is because I have to do aggregation on the local timezone, for example, :group => 'DATE(created_at)', GROUP BY DATE(created_at) will be based on the UTC, but I want to aggregate with one day in local timezone.
I knew how to write a migration file to migrate a certain datetime column. But there are a lot of such column, so I'm seeking for a better solution.

Answer 1

This is dangerous, but here is what I'd do in the migration:
class MigrateDateTimesFromUTCToLocal < ActiveRecord::Migration
def self.up
# Eager load the application, in order to find all the models
# Check your application.rb's load_paths is minimal and doesn't do anything adverse
Rails.application.eager_load!
# Now all the models are loaded. Let's loop through them
# But first, Rails can have multiple models inheriting the same table
# Let's get the unique tables
uniq_models = ActiveRecord::Base.models.uniq_by{ |model| model.table_name }
begin
# Now let's loop
uniq_models.each do |model|
# Since we may be in the middle of many migrations,
# Let's refresh the latest schema for that model
model.reset_column_information
# Filter only the date/time columns
datetime_columns = model.columns.select{ |column| [ :datetime, :date, :time].include? column.type }
# Process them
# Remember *not* to loop through model.all.each, or something like that
# Use plain SQL, since the migrations for many columns in that model may not have run yet
datetime_columns.each do |column|
execute <<-SQL
UPDATE #{model.table_name} SET #{column.name} = /* DB-specific date/time conversion */
SQL
end
rescue
# Probably time to think about your rescue strategy
# If you have tested the code properly in Test/Staging environments
# Then it should run fine in Production
# So if an exception happens, better re-throw it and handle it manually
end
end
end
end

Answer 2

My first advice is to strongly encourage you to not do this. You are opening yourself up to a world of hurt. That said, here is what you want:
class ShootMyFutureSelfInTheFootMigration
def up
Walrus.find_each do |walrus|
married_at_utc = walrus.married_at
walrus.update_column(:married_at, married_at_utc.in_time_zone)
end
end
def down
Walrus.find_each do |walrus|
married_at_local = walrus.married_at
walrus.update_column(:married_at, married_at_local.utc)
end
end
end
You may pass in your preferred timezone into DateTime#in_time_zone, like so:
central_time_zone = ActiveSupport::TimeZone.new("Central Time (US & Canada)")
walrus.update_column(:married_at, married_at_utc.in_time_zone(central_time_zone))
Or you can leave it and Rails will use your current timezone. Note that this isn't where you are, it is where your server is. So if you have users in Iceland and Shanghai, but your server is in California, every single 'local' time zone will be US Pacific Time.

Answer 3

Must you change the data in the database? Can you instead display the dates in local time zone. Does this help: Convert UTC to local time in Rails 3

Answer 4

Like a lot of other people said, you probably don't want to do this.
You can convert the time to a different zone before grouping, all in the database. For example, with postgres, converting to Mountain Standard Time:
SELECT COUNT(id), DATE(created_at AT TIME ZONE 'MST') AS created_at_in_mst
FROM users GROUP BY created_at_in_mst;