Ruby Time#to_json as a float

Question

Ruby's json library defaults to converting Time objects to Strings

require 'json'
Time.at(1000).utc.to_json # => "\"1970-01-01 00:16:40 UTC\"" 

The problem with this is that we lose precision. I'd like to_json to produce a float instead.

I also know there are some workarounds using oj or requiring json/add/time, but both of these add excess data to the output and aren't the most portable.

A straightforward approach is to monkey patch Time, although I'm not fond of doing that, especially to core classes

class Time
  def to_json(*a)
    self.to_f.to_json(*a)
  end
end

Are there any better approaches?

Answer 1

A straightforward approach is to monkey patch Time, although I'm not fond of doing that, especially to core classes

There's no JSON format for dates, as far as JSON cares they're just strings. Most languages understand ISO 8601, and that's what Time#to_json produces. So long as Time#to_json continues to produce an ISO 8601 datetime you'll remain backwards compatible.

require 'json'
require 'time'  # for Time#iso8601 and Time.parse

class Time
  def to_json
    return self.iso8601(6).to_json
  end
end

time = Time.at(1000.123456)
puts "Before: #{time.iso8601(6)}"

json_time = Time.at(1000.123456).to_json
puts "As JSON: #{json_time}"

# Demonstrate round tripping.
puts "Round trip: #{Time.parse(JSON.parse(json_time)).iso8601(6)}"

Before: 1969-12-31T16:16:40.123456-08:00
As JSON: "1969-12-31T16:16:40.123456-08:00"
Round trip: 1969-12-31T16:16:40.123456-08:00

---

If you're not comfortable with monkey patching globally, you can monkey patch in isolation by implementing around.

class Time
  require 'time'
  require 'json'

  def precise_to_json(*args)
    return iso8601(6).to_json(*args)
  end

  alias_method :original_to_json, :to_json
end

module PreciseJson
  def self.around
    # Swap in our precise_to_json as Time#to_json
    Time.class_eval {
      alias_method :to_json, :precise_to_json
    }

    # This block will use Time#precise_to_json as Time#to_json
    yield

  # Always put the original Time#to_json back.
  ensure
    Time.class_eval {
      alias_method :to_json, :original_to_json
    }
  end
end

obj = { 
  time: Time.at(1000.123456),
  string: "Basset Hounds Got Long Ears"
}

puts "Before: #{obj.to_json}"

PreciseJson.around {
  puts "Around: #{obj.to_json}"
}

puts "After: #{obj.to_json}"

begin
  PreciseJson.around {
    raise Exception
  }
rescue Exception
end

puts "After exception: #{obj.to_json}"

Before: {"time":"1969-12-31 16:16:40 -0800","string":"Basset Hounds Got Long Ears"}
Around: {"time":"1969-12-31T16:16:40.123456-08:00","string":"Basset Hounds Got Long Ears"}
After: {"time":"1969-12-31 16:16:40 -0800","string":"Basset Hounds Got Long Ears"}
After exception: {"time":"1969-12-31 16:16:40 -0800","string":"Basset Hounds Got Long Ears"}

Answer 2

You could save the number of nanoseconds since the Epoch.

require 'time'
require 'json'

t = Time.now
  #=> 2018-11-13 13:23:32 -0800
json = [t.to_i, t.nsec].to_json
  #=> "[1542144212,883611300]"

secs, nsecs = JSON.parse(json)
  #=> [1542144212, 883611300]
r = secs + 10**-9 * nsecs
  #=> (15421442128836113/10000000)
tt = Time.at r
  #=> 2018-11-13 13:23:32 -0800

t == tt
  #=> true

Note (10**-9).class #=> Rational.